// spreadsheets

What Is an .xlsx File?

An .xlsx file is a modern spreadsheet file format used by Microsoft Excel and many other spreadsheet programs. It can store worksheets, numbers, text, formulas, tables, charts, formatting and other structured spreadsheet data.

// definition

.xlsx is a spreadsheet file format

An .xlsx file is a spreadsheet workbook stored using Microsoft's Office Open XML spreadsheet format.

It is commonly associated with Microsoft Excel, although other spreadsheet applications can also read and write .xlsx files.

A single workbook can contain multiple worksheets along with formulas, formatting, charts and other spreadsheet information.

// file extension

What does .xlsx mean?

The .xlsx extension identifies an Excel workbook stored using the Office Open XML format.

budget.xlsx customers.xlsx sales-data.xlsx project-tracker.xlsx

The extension tells software what kind of file it is dealing with and which format should be used to interpret the contents.

// workbook

An .xlsx file is a workbook

A workbook can contain one or more worksheets.

sales.xlsx │ ├── January ├── February ├── March └── Summary

Each worksheet contains a grid of rows and columns.

// cells

Spreadsheet data is stored in cells

Each cell is identified by a column and row.

A1 B1 C1 A2 B2 C2

For example:

A	B	C
Product	Quantity	Price
Keyboard	2	39.99
Mouse	4	19.50

// cell values

Cells can contain different kinds of information

Text Numbers Dates Times Formulas Booleans Errors

// formulas

.xlsx files can store formulas

A spreadsheet cell does not have to contain only a fixed value.

It can contain a formula:

=A1+B1

or:

=SUM(B2:B20)

The spreadsheet application evaluates the formula and displays the calculated result.

// formula + value

A formula and its displayed result are different things

Suppose a cell contains:

=10+20

The displayed result may be:

30

The workbook can retain the formula rather than simply replacing it with the number 30.

// formatting

.xlsx files can store presentation information

Unlike a simple CSV file, an .xlsx workbook can retain substantial visual formatting.

Fonts Colours Borders Alignment Number Formats Column Widths Row Heights Conditional Formatting

// workbook features

A workbook can contain much more than cells

Worksheets

Multiple spreadsheet pages can exist inside one file.

Charts

Data can be visualised using charts and graphs.

Tables

Structured Excel tables can organise related records.

Formulas

Calculations can reference other cells and sheets.

Validation

Rules can restrict what users are allowed to enter.

Hyperlinks

Cells can contain links to websites and other resources.

// office open xml

An .xlsx file is based on XML

The modern Excel workbook format belongs to the Office Open XML family of file formats.

Internally, workbook information is divided across a collection of structured XML files and related resources.

Workbook ↓ XML documents + relationships + styles + other resources

// zip container

An .xlsx file is also a ZIP package

One of the most interesting facts about .xlsx files is that they are packaged using ZIP compression.

If a copy of an .xlsx file is renamed from:

workbook.xlsx

to:

workbook.zip

ZIP-compatible software can usually open the package and reveal the internal files.

An .xlsx file is not one giant XML document. It is a ZIP package containing multiple XML documents and supporting files organised according to the Office Open XML format.

// internal structure

What is inside an .xlsx file?

A simplified workbook package might contain:

workbook.xlsx │ ├── [Content_Types].xml ├── _rels/ │ ├── docProps/ │ └── xl/ │ ├── workbook.xml ├── styles.xml ├── sharedStrings.xml ├── worksheets/ │ ├── sheet1.xml │ └── sheet2.xml │ └── _rels/

The exact contents depend on which features are used in the workbook.

// worksheet xml

Worksheets are represented using XML

Internally, a worksheet may contain XML structures representing rows and cells.

<row r="1"> <c r="A1"> ... </c> </row>

Spreadsheet software interprets these structures and presents them as the familiar grid seen in Excel.

// shared strings

Text may be stored separately from worksheet cells

Some .xlsx workbooks use a shared strings table.

Instead of repeating the same text many times inside worksheets, cells can reference entries stored in a shared XML structure.

Cell ↓ String reference ↓ sharedStrings.xml ↓ "Freelance Coder"

// styles

Formatting can be stored separately

Workbook styles can be defined in internal style information and referenced by cells.

This avoids having to repeat complete formatting instructions for every individual cell.

// xlsx vs xls

.xlsx replaced the older .xls format

.xls	.xlsx
Older Excel format	Modern Excel workbook format
Binary file format	Office Open XML package
Associated with older Excel versions	Introduced with Excel 2007
Legacy format	Common modern format

// xlsx vs csv

.xlsx and .csv are very different

CSV	XLSX
Plain-text format	ZIP-based spreadsheet package
Usually represents one table	Can contain multiple worksheets
No native cell formatting	Supports extensive formatting
No native charts	Can contain charts
Simple data exchange	Complete spreadsheet workbook

// important distinction

Saving Excel data as CSV can remove workbook features

Suppose an .xlsx workbook contains:

3 worksheets formulas colours charts cell formatting

Exporting one sheet to CSV normally preserves the tabular values but not the complete workbook structure.

XLSX → full spreadsheet workbook CSV → plain tabular data

// macros

.xlsx files do not normally contain VBA macros

Excel uses a different extension for macro-enabled workbooks:

.xlsm

Extension	Workbook type
.xlsx	Standard Excel workbook
.xlsm	Macro-enabled Excel workbook

This distinction matters. If a workbook depends on VBA macros, saving it as an ordinary .xlsx file is not the appropriate format for retaining those macros.

// programming

.xlsx files can be created by software

Excel itself is not required to generate every .xlsx workbook.

Programming libraries can create, read and modify spreadsheet files automatically.

Python JavaScript PHP Java C# Automation

// python example

Python can create .xlsx files

For example, libraries such as openpyxl can work with Excel workbooks.

from openpyxl import Workbook workbook = Workbook() sheet = workbook.active sheet["A1"] = "Product" sheet["B1"] = "Price" sheet["A2"] = "Keyboard" sheet["B2"] = 39.99 workbook.save("products.xlsx")

Software can therefore generate spreadsheets from databases, APIs and other data sources.

// automation

Spreadsheet generation can be automated

API ↓ Application ↓ Process data ↓ Generate .xlsx ↓ Spreadsheet report

This is useful for reporting, financial data, exports, project tracking and business workflows.

// compatibility

.xlsx files are supported by many applications

Microsoft Excel is the application most strongly associated with .xlsx, but other spreadsheet software can also work with the format.

Microsoft Excel Google Sheets LibreOffice Calc Spreadsheet Libraries

Complex workbook features may not always behave identically when the same file is opened in different spreadsheet applications.

// not plain text

An .xlsx file is not a plain-text file

Although its internal structure contains XML documents, the .xlsx file itself is a compressed package.

Opening it directly in a basic text editor will therefore not display the workbook in the same clean way as opening a CSV, TXT or XML file.

TXT → plain text CSV → plain text XML → plain text XLSX → compressed package containing XML and other resources

// uses

What are .xlsx files used for?

Budgets Accounts Data Analysis Reports Project Tracking Invoices Customer Lists Dashboards Financial Models Data Exports

// mental model

The simplest way to think about .xlsx

.xlsx file ↓ ZIP package ↓ XML + relationships + resources ↓ Workbook ↓ Worksheets ↓ Rows + columns + cells ↓ Values + formulas + formatting

// summary

More than just a table.

An .xlsx file is a modern spreadsheet workbook format based on Office Open XML. It can contain multiple worksheets, values, formulas, formatting, charts and other spreadsheet features. Internally, the workbook is stored as a ZIP package containing XML documents and related resources.

// character encoding

What Is UTF-8?

UTF-8 is a character encoding used to represent text inside computers. It provides a way of turning Unicode characters such as letters, numbers, mathematical symbols and characters from many languages into bytes that a computer can store and process.

// definition

UTF-8 turns characters into bytes

UTF-8 stands for Unicode Transformation Format - 8-bit.

It is an encoding system that converts Unicode characters into sequences of bytes.

This allows text to be stored in files, transmitted across networks and interpreted consistently by different computer systems.

// why encoding?

Computers do not store letters directly

When we look at a computer screen, we see characters:

A £ π é 日 😀

But computer storage ultimately consists of binary data. The computer therefore needs a defined system for representing each character using numbers and bytes.

Character → Unicode code point → UTF-8 encoding → Bytes

// unicode

First, understand Unicode

Unicode is a standard that assigns characters unique numerical identifiers called code points.

A Unicode code point is normally written using U+ followed by hexadecimal digits.

Character	Unicode code point
A	U+0041
£	U+00A3
é	U+00E9
π	U+03C0
€	U+20AC
😀	U+1F600

// unicode vs utf-8

Unicode and UTF-8 are not the same thing

This distinction is important.

Unicode

Defines characters and assigns them code points.

UTF-8

Defines how those Unicode code points are encoded into bytes.

Unicode tells us that the pound sign is U+00A3. UTF-8 tells the computer which bytes should be used to represent that character.

// bytes

UTF-8 uses between one and four bytes

UTF-8 is a variable-length encoding.

Different characters require different numbers of bytes.

Character	UTF-8 bytes	Bytes used
A	41	1
£	C2 A3	2
π	CF 80	2
€	E2 82 AC	3
😀	F0 9F 98 80	4

Important: the hexadecimal byte values shown above are the UTF-8 encoding of each character, not the Unicode code point itself.

// ascii compatibility

UTF-8 is compatible with ASCII

One of the reasons UTF-8 became so successful is its compatibility with ASCII.

The first 128 Unicode characters use exactly the same single-byte values as ASCII.

A = U+0041 = UTF-8 byte 41 B = U+0042 = UTF-8 byte 42 1 = U+0031 = UTF-8 byte 31

This means ordinary English text using basic letters, digits and punctuation is very compact in UTF-8.

// multilingual text

UTF-8 can represent text from many languages

UTF-8 is not limited to English.

English: Hello Português: Olá Español: Información Français: Éducation Greek: π Japanese: 日本 Currency: £ € ¥ Mathematics: ≤ ≥ ± √ ∞ Emoji: 😀

All of these characters can exist within the same UTF-8 encoded document.

// encoding errors

What happens when the wrong encoding is used?

Text can become corrupted-looking when software interprets bytes using the wrong character encoding.

For example, a pound sign encoded as UTF-8 may sometimes incorrectly appear as:

Expected: £50 Incorrect interpretation: £50

This type of garbled text is often called mojibake.

The data may not actually be damaged. The same bytes may simply have been interpreted using the wrong encoding.

// txt files

UTF-8 and .txt files

A .txt file tells us that a file contains plain text, but it does not automatically tell us which character encoding was used.

.txt ≠ automatically UTF-8

A text file might use UTF-8, ASCII, Windows-1252 or another encoding.

For modern text files, UTF-8 is generally the most useful and interoperable choice.

// csv files

UTF-8 and CSV

CSV is also a text-based format, so character encoding matters.

Name,City,Amount João,Lisboa,£50 Élodie,Paris,€75 Sofia,Athens,€40

Saving this file using UTF-8 helps preserve accented characters and currency symbols when the file moves between compatible systems.

// xml

UTF-8 and XML

XML documents can explicitly declare their character encoding.

<?xml version="1.0" encoding="UTF-8"?>

This tells the software reading the document that its characters are encoded using UTF-8.

// html

UTF-8 on websites

Websites also need to specify how text should be interpreted.

Modern HTML pages commonly include:

<meta charset="UTF-8">

This tells the browser to interpret the document using UTF-8.

Correct encoding is important for displaying names, symbols, mathematical notation, international text and other characters reliably.

// advantages

Why is UTF-8 so widely used?

Unicode support

It can represent the full range of Unicode code points.

ASCII compatible

Basic ASCII characters retain their original one-byte representation.

Efficient

Common English characters require only one byte, while additional bytes are used when necessary.

Portable

UTF-8 is supported across operating systems, programming languages, databases and web technology.

// where it appears

UTF-8 appears throughout modern computing

UTF-8 can be used with many text-based technologies and file formats.

TXT CSV XML JSON HTML CSS JavaScript APIs Databases Web Pages

// key idea

The file format and the encoding are different things

This is one of the most useful concepts to understand.

File format: describes how information is organised.

Character encoding: describes how the characters themselves are represented as bytes.

CSV = structure of the data UTF-8 = encoding of the characters

A CSV file can therefore be encoded using UTF-8. An XML document can be encoded using UTF-8. A plain .txt file can also be encoded using UTF-8.

// summary

Characters become bytes.

UTF-8 is a character encoding that converts Unicode characters into sequences of one to four bytes. It allows computers to store and exchange text from many languages, symbols and writing systems while remaining compatible with ASCII.

// file formats

What Is XML?

XML is a text-based format for storing and exchanging structured information. It uses descriptive tags to show what each piece of data means and how different pieces of information relate to one another.

// definition

XML means Extensible Markup Language

XML stands for Extensible Markup Language.

It is a plain-text format designed to describe, organise, store and exchange structured information.

XML does this using tags that identify the meaning and structure of the data.

// example

What does XML look like?

Imagine that we want to store information about a customer.

<customer> <name>Alice Smith</name> <email>alice@example.com</email> <orders>4</orders> </customer>

The information is surrounded by tags such as <name>, <email> and <orders>.

These tags give meaning to the values rather than simply storing them in a fixed position.

// tags

Opening and closing tags

Most XML elements have an opening tag and a closing tag.

<name>Alice Smith</name>

Here:

<name>

This is the opening tag. It marks the beginning of the element.

</name>

This is the closing tag. The forward slash indicates the end of the element.

// structure

XML is hierarchical

One of XML's most important features is that elements can contain other elements.

This creates a tree-like hierarchy.

<shop> <customer> <name>Alice</name> <orders> <order> <id>1001</id> <total>49.99</total> </order> <order> <id>1002</id> <total>25.50</total> </order> </orders> </customer> </shop>

In this example, the shop contains a customer, the customer contains orders, and each order contains its own information.

// root element

An XML document needs a root element

A well-formed XML document has one outermost element that contains the rest of the document.

<customers> <customer> <name>Alice</name> </customer> <customer> <name>Ben</name> </customer> </customers>

Here, <customers> is the root element.

// attributes

XML elements can also have attributes

Attributes provide additional information about an element.

<customer id="1042" status="active"> <name>Alice Smith</name> </customer>

In this example:

id="1042" status="active"

These are attributes attached to the customer element.

// extensible

Why is XML called "extensible"?

XML does not give you a fixed set of tags.

Instead, you can create tags appropriate to the information you are describing.

<book> <title>Introduction to Data</title> <author>Jane Smith</author> </book>

A different system might use:

<product> <sku>ABC-100</sku> <price>29.99</price> </product>

The tags can therefore be designed around the type of information being stored.

// declaration

The XML declaration

XML documents may begin with a declaration such as:

<?xml version="1.0" encoding="UTF-8"?>

This tells software that the document uses XML version 1.0 and that the text is encoded using UTF-8.

Important: XML is a text format, so character encoding matters. UTF-8 is extremely common because it can represent characters from many languages and writing systems.

// escaping

Some characters need special handling

Because characters such as angle brackets are used as part of XML syntax, certain characters must be represented using special entity references when they are intended as data.

Character	XML representation
<	&lt;
>	&gt;
&	&amp;
"	&quot;
'	&apos;

// rules

XML has strict structural rules

XML must be well formed for an XML parser to process it correctly.

Tags must close

Elements normally need corresponding opening and closing tags.

Tags must nest correctly

Elements cannot overlap one another incorrectly.

One root element

The document must have one outermost root element.

XML is case-sensitive

<Name> and <name> are treated as different tags.

// nesting

Correct nesting matters

This is valid:

<customer> <name>Alice</name> </customer>

But this structure is invalid:

<customer> <name>Alice </customer> </name>

The elements have been closed in the wrong order.

// xml vs html

XML and HTML look similar, but they have different purposes

Both use angle-bracket tags, but they are designed for different jobs.

XML	HTML
Describes and structures data	Structures content for web pages
Custom tags can be created	Uses predefined HTML elements
Focused on information	Focused on webpage structure and presentation
Strict syntax	Browsers are often more forgiving

<!-- XML --> <product> <name>Keyboard</name> <price>49.99</price> </product> <!-- HTML --> <h1>Keyboard</h1> <p>Price: £49.99</p>

// xml vs json

XML and JSON can represent similar information

XML and JSON are both commonly used to represent structured data.

For example, this XML:

<customer> <name>Alice</name> <orders>4</orders> </customer>

could be represented in JSON as:

{ "customer": { "name": "Alice", "orders": 4 } }

JSON is often more compact, while XML offers features such as attributes, namespaces and document-oriented structures that can be useful in more complex systems.

// uses

What is XML used for?

XML has been used extensively for exchanging and storing structured information across many kinds of software.

Configuration Files Data Exchange Web Services Document Formats Product Feeds Sitemaps RSS Feeds APIs Data Imports Data Exports

// real-world example

Website sitemaps commonly use XML

A website can provide an XML sitemap containing information about its pages.

<urlset> <url> <loc>https://example.com/</loc> </url> <url> <loc>https://example.com/about</loc> </url> </urlset>

Search engines and other software can process this structured information automatically.

// data management

Why XML matters in data management

XML allows information to describe its own structure.

Instead of relying purely on column positions or an external explanation, tags can identify what individual pieces of information represent.

<employee> <name>Tiago</name> <department>Technology</department> <active>true</active> </employee>

This makes XML useful when information needs to move between systems while retaining a clear hierarchical structure.

// plain text

XML is still plain text

Although XML can represent complex structures, the file itself is text.

This means an XML file can be opened in a text editor, inspected by a person and processed by software.

Readable Portable Structured Machine-Readable Text-Based

// summary

Text with structure and meaning.

XML is a text-based language for representing structured information. It uses tags, elements, attributes and hierarchical relationships to describe what data means and how different pieces of information fit together.

// file formats

What Is a .txt File?

A .txt file is one of the simplest ways of storing information on a computer. It contains plain text without the complex formatting found in documents, spreadsheets or web pages.

// definition

Plain text, stored in a file

A .txt file is a plain-text file. The letters .txt are the file extension, which tells a computer that the file primarily contains text.

Unlike a Microsoft Word document, a .txt file does not normally contain information about fonts, colours, page layouts, images or complex formatting.

It is essentially a sequence of characters stored in a file.

// example

What does a .txt file contain?

A text file might contain something as simple as:

> Customer: Jane Smith Order Number: 10452 Product: Laptop Stand Quantity: 2 Status: Dispatched

There is no special document structure required here. These are simply characters and line breaks stored inside a file.

// why use txt?

Simple files can be extremely useful

Lightweight

Text files usually require very little storage because they contain little more than the characters themselves.

Widely supported

Almost every operating system and programming language can read and write plain-text files.

Easy to inspect

A person can open a .txt file directly and see the information without needing specialist software.

Easy to process

Programs and scripts can search, transform and analyse text files very efficiently.

// data

A .txt file can contain many kinds of information

The .txt extension does not dictate exactly what the text must represent.

A text file could contain notes, lists, configuration information, source data, logs, mathematical expressions, website content or information intended to be processed by another program.

Notes Lists Logs Data Code Configuration URLs Mathematics

// encoding

What does UTF-8 have to do with .txt files?

Computers ultimately store information as numbers. A text encoding defines how those numbers correspond to characters such as letters, numbers and symbols.

UTF-8 is one of the most widely used character encodings. It can represent ordinary English text as well as characters from a huge range of languages and writing systems.

> English Português Mathematics: π ≈ 3.14159 Currency: £50 Symbols: ≤ ≥ ± √

Important: a file ending in .txt is not automatically guaranteed to use UTF-8. Text files can use different character encodings.

When exchanging modern text data, UTF-8 is usually an excellent choice because it supports such a wide range of characters.

// txt vs document

A text file is not the same as a formatted document

Suppose you write:

Hello, world!

A .txt file mainly stores those characters.

A word-processing document may also store information describing the font, font size, colour, margins, paragraph spacing, images and many other formatting properties.

This is why plain-text files are often much simpler to process programmatically.

// related formats

Many important file formats are text-based

Plain text is also the foundation of many structured formats used throughout computing and the web.

.txt .csv .json .xml .html .css .js

These formats add their own rules and structures, but the underlying content can still be represented as text.

This is one reason understanding plain text is so useful when working with data, websites, APIs and automation.

// practical use

When is a .txt file useful?

Saving simple information

Notes, instructions, lists or other information that does not require visual formatting.

Moving data

Text can be transferred between different applications and computer systems with relatively little complexity.

Programming

Scripts can create, read, search and modify text files easily.

Automation

Text files can act as simple inputs, outputs, logs or intermediate data in automated processes.

// summary

Simple format. Huge usefulness.

A .txt file is simply a file containing plain text. Its simplicity makes it one of the most universal, portable and useful ways of storing information on a computer.