Where a datum feature symbol is placed in line with a dimension line or on a feature control frame associated with a feature of size, the entire feature of size is the datum feature.

Consider this sentence. ''The dog walked I.'' We know something is wrong: either the order is incorrect or the personal pronoun is the incorrect part of speech. Depending on your assumption, you might interpret this sentence in one of two ways, ''I walked the dog'' or ''The dog walked me.''

In the same way, when datum features are incorrectly sequenced, the tolerance is ambiguous. I frequently see drawings that have datum features specified in feature control frames in an order of precedence that makes no sense. Proper datum sequencing is extremely important to insure that only one, correct interpretation is possible. The attached excerpt from my book will help clarify this subject.

All datum features must be specified in order of precedence. Each datum feature must be identified with a datum feature symbol. Datum feature symbols must not be applied to centerlines, center planes, or axes.

I often see drawings with datum feature symbols attached to axes or center planes, and I sometimes have difficulty convincing engineers that it is impossible for inspectors to orient or locate features relative to imaginary lines and planes. The discussion on the attached file will help clarify this issue.

Symbols, terms, and rules are the basics of geometric dimensioning and tolerancing. They are the alphabet, the definitions, and the syntax of this language. The GD&T practitioner needs to be very familiar with these concepts and how to use them. This month's article includes the 16 GD&T terms that I consider to be essential to understand the current geometric tolerancing standard.