# Creating Words

One of Forth's most powerful features is the ability to define new words. A word definition gives a name to a sequence of operations. Once defined, you can use the new word just like any built-in word.

## The Syntax

Use `:` to start a definition and `;` to end it:

```forth
: double dup + ;
```

This creates a word called `double` that duplicates the top value and adds it to itself. Now you can use it:

```forth
3 double    ;; leaves 6 on the stack
5 double    ;; leaves 10 on the stack
```

The definition is simple: everything between `:` and `;` becomes the body of the word.

## Definitions Are Shared

When you define a word in one step, it becomes available to all other steps. This is how you share code across your pattern. Define your synths, rhythms, and utilities once, then use them everywhere.

Step 0:
```forth
: bass "saw" s 0.8 gain 800 lpf ;
```

Step 4:
```forth
c2 note bass .
```

Step 8:
```forth
g2 note bass .
```

The `bass` word carries the sound design. Each step just adds a note and plays.

## Redefining Words

You can redefine any word, including built-in ones:

```forth
: dup drop ;
```

Now `dup` does the opposite of what it used to do. This is powerful but dangerous. Redefining core words can break things in subtle ways.

You can even redefine numbers:

```forth
: 2 4 ;
```

Now `2` pushes `4` onto the stack. The number two no longer exists in your session. This is a classic Forth demonstration: nothing is sacred, everything can be redefined. 

## Practical Uses

**Synth definitions** save you from repeating sound design:

```forth
: pad "sine" s 0.3 gain 2 attack 0.5 verb ;
```

**Transpositions** and musical helpers:

```forth
: octup 12 + ;
: octdn 12 - ;
```

## Words That Emit

A word can contain `.` to emit sounds directly:

```forth
: kick "kick" s . ;
: hat "hat" s 0.4 gain . ;
```

Then a step becomes trivial:

```forth
kick hat
```

Two sounds, two words, no clutter.

## Stack Effects

When you create a word, think about what it expects on the stack and what it leaves behind. The word `double` expects one number and leaves one number. The word `kick` expects nothing and leaves nothing (it emits a sound as a side effect). Well-designed words have clear stack effects. This makes them easy to combine.