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# Undefined control sequence

This error appears when LaTeX does not understand one of the commands you have used.

## Common Examples

**Typo in a command:**

The most common causes of such an error are simple typos. An example of such a typo is shown below, where you accidentally pressed **`Z`** instead of **`a`** when writing **`\alpha`**

```latex
A typo when writing $\alpha$ could be $\Zlpha$
```

This will give an error message of

main.tex, line 10

Undefined control sequence.

< \Zlpha l.10 A typo when writing $$\alpha$$ could be $\Zlpha $ The control sequence at the end of the top line of your error message was never \def'ed. If you have misspelled it (e.g., \`\hobx'), type \`I' and the correct spelling (e.g., \`I\hbox'). Otherwise just continue, and I'll forget about whatever was undefined. \[1

These errors are easily spotted by humans but can cause LaTeX to get confused as to what is being asked.

**Forgetting to load a package:**

Another cause of such an error is when a specific package is needed to use a certain command, but it is accidentally forgotten in the preamble. An example of this would be

```latex
I want to include a space after the word \LaTeX\xspace but I have forgotten to load the xspace package.
```

In this example, the document will fail to compile as LaTeX doesn't recognize the **`\xspace`** command. This is not a typo, as **`\xspace`** is a perfectly fine command. The problem is that you have forgotten to include **`\usepackage{xspace}`** in the preamble. When this line is included, the error message will disappear as LaTeX now knows how to interpret the **`\xspace`** command.

**Backslash used in wrong place:**

Another cause of an *Undefined Control Sequence* error is a backslash used inappropriately. This can happen particularly when writing file links as shown below.

```latex
An error will be generated if you write a file path as

C:\Users\Files
```

The issue is that when LaTeX sees a backslash **`\`**, it interprets what follows as a command. Here, there is no such command as \Users, so you will get an *Undefined Control Sequence* error. To avoid this, when writing text you should write a backslash as **`\backslash`**. For writing long file paths and urls, it may sometimes be more convenient to use the **`url`** package rather than writing **`\backslash`** every time.


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