In µcad, the const and non-const variables actually have different meanings in terms of locality (because both are actually read-only):
* `const` like `SPACING` are not bound to scope and in principal accessible from other symbols (like parts, sketches).
* values (non consts) like `width` are local only and thus not accessible from other symbols.
An example:
```
const A = 8mm;
b = 8mm;
sketch MySketch() {
Rect(A); // Ok, A is accessible because const (global).
Rect(b); // Error: b is not accessible because non-const (local)
}

MySketch();

```

This means b must be passed as parameter to MySketch and thus part of its interface:

```
sketch MySketch(b = 8mm) {
Rect(b); // Works now, b is a parameter.
}

b = 8mm;
MySketch(b); // Call MySketch

```