At the end of the 4th fetal month, the suprapatellar septum completely separates the knee joint cavity from the suprapatellar bursa. A perforation of the septum normally occurs at the end of the 5th fetal month. Mechanical factors, such as the pressure and friction of the quadriceps tendon on the condylar surface, are said to determine the form and extent of the perforation. Anatomic studies of adult knee joints revealed four variants in the further development of the suprapatellar septum, namely, a completely preserved septum (complete septum), a perforated septum with the communication between suprapatellar bursa and knee joint cavity differing in localization and number (perforated septum), a residual septum in the form of a fold that is almost always in a medial localization (residual septum), or a completely involuted septum (extinct septum). The clinical term "suprapatellar synovial plica" should not be used as a synonym for the expression "suprapatellar septum." The suprapatellar synovial fold corresponds to the residual septum and thus is only one of the four types of the suprapatellar septum.