先相先 (sen-ai-sen) is a Japanese go term which describes the handicap of taking Black in two games out of every three. This was the smallest handicap possible between two players in the era before the introduction of komi .
Usually, after three games are won in a row by the same player (or some other threshold is reached) the handicap shifts to josen or to tagaisen (even). A game which if lost would result in a shift in the handicap is called a kadoban.
 On average, sen-ai-sen gives Black a 1/6 move's advantage.