screen-internal-geometries returns the internal geometries of all the "monitors" of screen. A "monitor" typically corresponds to a physical monitor, but can be anything that the underlying GUI system considers a monitor.
The internal geometry of a monitor is a rectangle which excludes "system areas" like taskbars and global menu bars and so on. Examples of these include the Windows taskbar, the Mac OS X menu bar, and the Mac OS X Dock. See screen-internal-geometry for information about displaying CAPI windows in system areas.
Each internal geometry is represented as a screen rectangle. A screen rectangle is a list of four numbers: x and y being the coordinates as offsets from the top-left of the primary monitor, and width and height .
On GTK+ when using a desktop with separate workspaces, the workspaces may be considered as separate "monitors". When there are multiple real monitors, the values may be incorrect. You can use screen-monitor-geometries to check the number of monitors, and to check the full size of the monitors.
CAPI User Guide and Reference Manual (Macintosh version) - 3 Aug 2017