Thymine can form four dimers. The one shown here is the one called syn-cis. There is a choice as to whether the thymines are aligned in the same direction (syn) or in opposite directions (anti) when they approach. The other choice is whether the two thymines will lie on top of each other or not. If the former is true then the heterocyclic rings will be on the same side of the cyclobutane ring (cis) and otherwise they will be trans. The syn-cis dimer is not the most stable of the dimers (think about it) but it is formed in DNA because the thymines are aligned in DNA in a fashion that forces the formation of this kind of dimer.