The Trefoil is the fifth lake in a row according to the official numbering of the Seven Rila Lakes.

The lake is irregularly shaped and forms something like three fingers, hence its name. In the recent past, the lake was called Sredniya gol̀.

At the end of October, the surface of the lake freezes and is covered with a thick layer of ice that can reach two meters in thickness. In June, the ice melts. As the ice melts, additional small lakes and streams appear in various places around the mountain, and this often happens with larger lakes.

It is located at 2216 m above sea level. It has an irregular shape and low banks. Its area is 2.6 ha, it is 6.5 m deep.