102 Geometric Dimensioning and Tolerancing Datum Axis A cylindrical object can be a datum feature. When the cylindrical datum feature is used, the center axis is known as the datum axis. There are two theoretical planes intersecting at 90°. These planes are represented by the centerlines of the drawing. Where these planes intersect is referred to as the datum axis. The datum axis is the origin for related dimensions, while the X and Y planes indicate the direction of measurement. A datum feature symbol is added to the end of the object to establish the datum reference frame, as shown in Example 3-28. The Drawing The Meaning X Y Y Secondary datum axis X Part Primary datum Secondary datum axis Example 3-28. Datum axis.