window-left-column

window-left-column is a built-in function in `C source code'.

(window-left-column &optional WINDOW)

Return left column of window WINDOW.
This is the distance, in columns, between the left edge of WINDOW and
the left edge of the frame's window area. For instance, the return
value is 0 if there is no window to the left of WINDOW.

WINDOW must be a valid window and defaults to the selected one.