Four synovial plicae are commonly recognised:
The filmy band is actually a fold in the capsule of the knee, but that is not appreciated in this illustration.
This illustration gives a better example of how the plica folds in from the capsule surrounding the knee joint.
Synovial Plica Syndrome of the Knee: A Commonly Overlooked Cause of Anterior Knee Pain. Lee et al. Surg J (N Y). 2017 Jan; 3(1): e9–e16.
Medical Plica Syndrome Sznajderman T. et al. IMAJ 2009: 1: January: 54-57.
Synovial Plicae in the Knee Boles CA and Martin DF. American Journal of Roentgenology. 2001;177: 221-227.