When a yavam and a yevama consummate the relationship, that's yibum and now they're married. If they do chalitzah, they're done. But that seems to be only when either action is taken initially, and when not complicated by other actions. 3 divergent opinions on the different permutations here. Also, a new chapter, and the unexpected cases of a man who sleeps with a woman without knowledge or intent, and various permutations of that.