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G21 G-code
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: