Menus > Graph Menu > Mark Coordinate System

Selection prerequisites: A coordinate system’s axis, origin point, unit point, unit circle, or grid

This Graph menu command marks the coordinate system associated with the selected object as the coordinate system on which to measure or plot new objects.

When you have multiple coordinate systems, one of those coordinate systems is the marked coordinate system. All of the Graph menu commands apply to the marked coordinate system, as do those Measure menu commands that require a coordinate system.

The marked coordinate system is normally the most recently created coordinate system; you use this command to mark a different coordinate system.

Normally you’ll have only a single coordinate system, and you won’t have to worry about which coordinate system is marked.