to cross one's t's (and dot one's i's) "to be exact" is attested from 1849. Phrase to a T "exactly" is recorded from 1690s, though the exact signification remains uncertain despite much speculation. The measuring tool called a T-square (sometimes suggested as the source of this) is recorded by that name only from 1785. The T-cell (1970) so called because they are derived from the thymus. As a medieval numeral, T represented 160.