PIE base ‘to stretch, extend, span’.
Derivatives include tone, intend, contain, entertain, stretch, spread, extend, hypotenuse, and perhaps thing, originally stretch of time for an assembly.
Also root of Sanskrit tantra, a spiritual practice for unifying opposites in Wholeness, as in Divine lovemaking, for instance, from Sanskrit tantram ‘loom’, unifying ‘warp’ and ‘weft’, from tan ‘to stretch’, and -tra-m ‘instrument’. So tantra literally means ‘an instrument for stretching’.
[Pokorny *ten-1, tend-, pp. 1065–1066.]