This plastic sash cam fits onto the upper top sides of the window (vent) that is sliding up or down. The sash cam and window then rest on the top guide of the sash balance. It has dimensions of top being 1 in. wide by 5/8 in, deep and 1-5/16 in. tall. Width of this top guide needs to fit correctly in the window's track frame in order for it to slide up or down properly, and the depth allows the window to correctly rest on the top guide of the sash balance. 

sash cams are used in hung windows that use channel balances | they mount to the top side edges of the moving vent, 2 per window | they actually engage and push the channel balance top guide, moving the balance disabled