## atan

**atan**is a built-in function in `

`C source code`

'.(atan

__Y__&optional

__X__)

Return the inverse tangent of the arguments.

If only one argument

__Y__is given, return the inverse tangent of

__Y__.

If two arguments

__Y__and

__X__are given, return the inverse tangent of

__Y__

divided by

__X__, i.e. the angle in radians between the vector (

__X__,

__Y__)

and the x-axis.