Uracil can form four dimers. The one shown here is the one called syn-cis. There is a choice as to whether the uracils are aligned in the same direction (syn) or in opposite directions (anti) when they approach. The other choice is whether the two uracils will lie on top of each other or not. If the former is true then the uracils will be on the same side of the cyclobutane ring (cis) and otherwise they will be trans.