Gangable Switch Box:
A gangable switch box is a modular box having removable sides allowing a single box to be expanded into a 2-gang, 3-gang or 4-gang box holding multiple devices.
The boxes are characterized by removable sides having a screw at one end, and on the other end, a tab that interlocks the removable side with the box body. The orientation of the screw and interlocking tab flip from the left side of the box to the right side of the box. In other words, the screw will be on the bottom left side as in this picture, and also will be found on the top right side of this box.
With gangable "new work" boxes, holes are provided in the side panels at the back of the box for nailing the box to the wood stud if used as a single gang box. Sometimes these new work boxes will come with a tab or bracket attached to one side for fastening to the wood stud when using box in single or multiple gang box configurations.