# Units

We start with an optional `units`

clause, where we can define units to use in relative calculations.

The four predefined ones are `U`

, `u`

, `cx`

, and `cy`

.

`U: 19.05 # 19.05 MX spacingu: 19 # 19mm MX spacingcx: 18 # 18mm Choc X spacingcy: 17 # 17mm Choc Y spacing`

But we can add any other (or modify these predefined ones), or even use an existing measure in calculating a new value (for example, `double: 2 u`

).
Recall how each string that can be interpreted as a math formula will be treated like a number, so this is a great way to add math-level variables to your config.

`units: a: cy - 7 b: a * 1.5`