sub·tend

– verb
1. to extend under or be opposite to.
2. to occur beneath or close to.
3. to form or mark the outline or boundary of.

sur·face

– noun
1. the outer face, outside, or exterior boundary of a thing; outermost or uppermost layer or area.
2. any face of a body or thing.
3. extent or area of outer face; superficial area.
4. the outward appearance, esp. as distinguished from the inner nature.
5. any figure having only two dimensions; part or all of the boundary of a solid.
– adj.
6. apparent rather than real; superficial.
7. of, pertaining to, or via land or sea.
8. belonging to a late stage in the transformational derivation of a sentence; belonging to the surface structure.