Constraint?

An image of a gap style go nogo gauge, for external
                  measurements (such as the diameter of a shaft being produced by a lathe). It looks
                  sort of like a small C-clamp with two jaws (instead of just
                  one).Photograph taken by Glenn McKechnie, 2005-03-26, available via CC-by-sa-2.0 license.

Customizing the TEI Schema, slide 2 of 14