image http://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/English_length_units_graph.svg/605px-English_length_units_graph.svg.png style="width:300px"

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.

Format

The command is used as such :

G21

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

Parameters

No parameters.

Further reading

These resources are used as references for Gcode :