The fifth chapter! Plus, the options of ma'amar, get, chalitzah... and, of course, yibum. And how the may follow one from the other, as it happens. Note - a long mishnah here establishes the basic cases and basic principles - for example, once ma'amar has been done, it can't be done again in that same context. The Gemara basically establishes where Rabban Gamliel and the sages agree and disagree.