ISO 5459:2011 - Placement of a datum feature indicator as an extension of an angular dimension line. This enables the designer to use the median line of a cone as a datum line and the median (bisector) plane of a wedge as a datum plane. Wedges and cones are very common features in plastic parts, metal cast parts and stamped parts. Cylinders become cones when drafted, likewise blocks become wedges. Designers are seriously handicapped if these features cannot be used in datum systems.
See ISO 5459:11 Example C.1.5.
