sub·tend 
(səb-tĕnd)
(səb-tĕnd)tr.v. sub·tend·ed, sub·tend·ing, sub·tends
1. Mathematics To be opposite to and delimit: The side of a triangle subtends the opposite angle.
2. To underlie so as to enclose or surround: flowers subtended by leafy bracts.
[Latin subtendere, to extend underneath : sub-, sub- + tendere, to extend; see ten- in the Appendix of Indo-European roots.]