G21 means “all moves starting now have their units specified in millimeters ( metric units )”.

G21's evil twin is G20, which means “all moves starting now have their units specified in inches ( imperial units )”.

An inch is 25.4 millimeters.

G21 is the default when Smoothie starts.

The command is used as such :

G21

Which means : units are now metric until another G-code changes it ( modal command )

No parameters.

These resources are used as references for Gcode :

• LinuxCNC Gcode list
• Reprap Gcode list