ten-
“to stretch, to pull”stretch, draw, pull
Root for stretching, yielding Latin tendere, Greek teinein, English thin, tone.
Discussion
The PIE root *ten- (to stretch, to pull, to draw tight) produced one of the largest derivative families in the IE vocabulary — a word clan that connects the physical act of stretching fiber to the abstract concepts of tension, attention, tendency, and the thin.
Latin tendere (to stretch, to extend) and its variant tenēre (to hold, to keep — originally "to keep stretched") are the twin Italic reflexes, and together they generated an extraordinary English vocabulary. From tendere: tend, tendency, tender (stretched thin, hence delicate), tense (stretched, hence strained — both the muscular and psychological senses), tension, tent (a stretched fabric shelter), extend (to stretch out), distend (to stretch apart), pretend (to stretch forth — to put forward a claim), contend (to stretch against — to strive), attend (to stretch toward — to pay attention), and the musical term tenor (the voice that "holds" the melody).
From tenēre: tenant (one who holds), tenure (the act of holding), tenacious (holding tightly), contain (to hold together), content (held within), continent (the land that holds together), continue (to hold on), detain (to hold back), maintain (to hold by hand), obtain (to hold against — to secure), retain (to hold back), and sustain (to hold from below).
Greek teínein (τείνειν, "to stretch") gave: tone (tónos, "a stretching" — the tension of a vibrating string, hence a musical note), hypertension (excessive stretching/pressure), and the combining form tono-.
English thin (OE þynne, from PGmc *þunnuz, from PIE *tn̥-u-, the zero-grade of *ten-) means "stretched out" — what is pulled thin has been stretched. The word preserves the most physical sense of the root.
Sanskrit tanóti ("he stretches") and the related tantra ("loom, warp" — then "doctrine, system" — the weave of religious teaching) confirm the Indo-Iranian reflex. The tantric tradition is etymologically the tradition of the stretched/woven teaching.
Notes
Pokorny 1065-1066. English tend, tense, tent, tendon, tone, thin.