A datum reference or just datum (plural: datums) is some important part of an object—such as a point, line, plane, hole, set of holes, or pair of surfaces—that serves as a reference in defining the geometry of the object and (often) in measuring aspects of the actual geometry to assess how closely they match with the nominal value, which may be an ideal, standard, average, or desired value. For example, on a car's wheel, the lug nut holes define a bolt circle that is a datum from which the location of the rim can be defined and measured. This matters because the hub and rim need to be concentric to within close limits (or else the wheel will not roll smoothly). The concept of datums is used in many fields, including carpentry, metalworking, needlework, geometric dimensioning and tolerancing (GD&T), aviation, surveying, geodesy (geodetic datums), and others. 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 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 specified range of weight and (chiefly longitudinal) balance; an airman is responsible for determining these factors for each flight under his or her command. This requires the calculation of 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 longitudinal axis, and specified in its operating handbook. Engineering An engineering datum used in geometric dimensioning and tolerancing is a feature on an object used to create a reference system for measurement. In engineering and drafting, a datum is a reference point, surface, or axis on an object against which measurements are made. These are then referred to by one or more 'datum references' which indicate measurements