The geometry that you select must result in a valid 1-dimensional measurement. That is, you must define a measurement that results in an unambiguous result. For example, you cannot select nonparallel planes because the distance between planes is ambiguous (the distance depends on the point at which you are measuring to on each plane).

The following table shows the combinations of geometry references that are valid for defining a 1-dimensional tolerance analysis measurement.

Once you have selected the measurement references, the application may prompt you to select a measurement direction feature. If you select a planar surface or a datum plane, the measurement direction is normal to the selected plane. If you select a linear edge or datum axis, the direction is along the selected edge or axis. When the direction of the measurement can be derived from the measurement references (for example, one or both references are planes), you do not need to select a separate direction feature.