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. The weaker player takes Black, White and Black in this order. This was the smallest handicap possible between two players in the era before the introduction of komi . For more information on professional handicaps in the days before komi go, see Match Handicap System.
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.
In still older times, the weaker player used to take Black, Black and White in this order. This was called 先先先 (Sen-zen-sen).
 On average, sen-ai-sen gives Black a 1/6 move's advantage.