to a T

• down to a T (emphatic)
• to a tee

The origins of this phrase are uncertain, but it has been observed in print since at least 1766, and likely was around well before that. The potentially related phrase "to a tittle" is found in a 1607 play, The Woman Hater by Francis Beaumont and John Fletcher ("I'll quote him to a tittle"). The T in the phrase to a T is likely the first letter of a word, with tittle being the most likely source.

• Other theories with little evidence point to golf tees, for their small size; this may have at least influenced the alternate form to a tee. Some speculate a relationship with T-square, a measuring device introduced around the turn of the century. Others claim the expression refers to the correct completion of the letter t by crossing it.
• In print from "Two Years Before the Mast" published in 1840, and, even then, using quotes, refers to the practice of squaring up a yardarm with a mast on a sailing ship such that it made a perpendicular T.

to a T

1. (idiomatic) Precisely; exactly; perfectly; with great attention to detail.

   The announcement of the political endorsement was timed to a T.

   • 1766, George Colman & David Garrick, The Clandestine Marriage, Act iii, Scene 2.

     Mrs. Heidel. My ſpurrit to a T.

   • 1868, Ann Sophia Stephens, Doubly False

     That accounts for my having the dress, but it don't account for the piece that you left sticking to the rose-bush under Mrs. Lander's bed-room winder, which piece I took off that morning, and which piece I matched with the dress after you pitched it at me over them bannisters; it was an awful scragly tear, and it fitted to a T.

• atto-, tota