Editing Datum reference (section)

==Uses==
In carpentry, an alternative, more common name is "face side" and "face edge". The artisan nominates two straight edges on a [[workpiece]] as the "datum edges", and they are marked accordingly. One convention is to mark the first datum edge with a single slanted line (/) and the second with double lines (//). For most work, the datum references of the workpiece need to be [[Perpendicular|square]]. If necessary they may be cut, planed or filed to make them so. In subsequent marking out, all measurements are then taken from either of the two datum references.

In aviation, an aircraft is designed to operate within a [[flight envelope|specified range]] of mass and (chiefly longitudinal) [[Center of gravity of an aircraft|balance]]; an airman is responsible for determining these factors for each flight under their command. This requires the calculation of [[Moment (physics)|moment]] for each variable mass in the aircraft (fuel, passengers, cargo, etc.), by multiplying its weight by its distance from a datum reference. The datum for light airplanes is usually the engine firewall or the tip of the spinner, but in all cases it is a fixed plane perpendicular to the aircraft's [[Aircraft principal axes|longitudinal axis]], and specified in its operating handbook.