In GD&T the term feature-of-size (FOS) refers to any surface, or set of parallel surfaces associated with a size dimension. Specific examples of features of size include:
- A hole diameter (a cylindrical surface)
- Plate thickness (two opposed parallel surfaces)
- Peg diameter (another cylindrical surface)
- A ball bearing diameter (a spherical surface)
The example below shows three views of a plate with two holes. There are two dimensions called out in this drawing: a plate thickness and a hole diameter. Either of these is considered a FOS.