0080// What is the difference between A parallel to B and B parallel to A ?

[Courtesy: by Jens Rost]

In math, you can confidently say that, A parallel to B same as B parallel to A. However, in GD&T perspective, this is not the case. Why?

Consider A and B is a planar features, A have called out parallelism 5 micron with respect to B. In this case, B is a datum feature and A is a feature we intended to control it's form and orientation with parallelism. This mean, datum plane will establish by using physical datum feature simulator to simulate the datum feature B. Therefore, simulated datum plane A is derived by "high points" from physical datum feature simulator which contact the datum feature A. Detail can refer to Y14.5-2009, Fig. 4-10 Datum Plane Establishment.

Sound confusing? Let see the picture below for better visualization.

The logic to establish datum for B which have called out parallelism 5 micron with respect to A is exactly the same.

However, is there any difference between these two scenarios? The answer is, YES !!!

From the method of datum plane establishment, you can clearly see that. datum is a perfect plane for establish the orientation reference system (more generic term in GD&T, can call it datum reference frame). But, the feature which we want to control is a non-planar elements.

When you perform non-continuous measurement (measurement with finite resolution), information for this non-planar elements represent as a set of points with coordinates information. At the same time, datum plane which established is a perfect plane which can defined by plane equation. By comparing these two nature of information, we can clearly differentiate they are not equivalent. Still cannot visualize? Below is the diagram in 2D space for better explanation.