What is Cujus Est Divisio, Alterius Est Electio?

Legal Definition
Whichever [of two parties] has the division, [of an estate,] the choice [of the shares] is the other's. Co. Litt 166b. In partition between coparceners, where the division is made by the eldest, the ruie in English law is that she shall choose her share last. Id.; 2 Bl. Comm. 189; 1 Steph. Comm. 323.
-- Black's Law Dictionary