An Etymological Dictionary of Astronomy and Astrophysics

To be opposite and delimit the extent of an angle or side of a geometric figure.

Etymology (EN): From L. subtendere “to stretch beneath,” from → sub-

• tendere “to stretch,” cognate with Pers. târ “string,” tanidan “to weave,” as below.

Etymology (PE): Zirtânidan from zir-, → sub-, + tânidan from tân “thread, warp of a web,” tâl “thread” (Borujerdi dialect), târ “thread, warp, string,”
related to tur “net, fishing net, snare,” from tanidan, tan-
“to spin, twist, weave;” Mid.Pers. tanitan; Av. tan- “to stretch, extend;” cf. Skt. tan- to stretch, extend;" tanoti “stretches,” tántra- “warp; essence, main point;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch, as above; Lith. tiñklas “net, fishing net, snare,” Latv. tikls “net;” PIE base *ten- “to stretch.”