Two features distinguish the siamang from other gibbons. First, two digits on each foot—the second and third toes—are partially joined by a membrane—hence the specific name syndactylus, from the Ancient Greek σύν, sun-, "united" + δάκτυλος, daktulos, "finger". Second, a large gular sac (throat pouch), found in both males and females of the species, can be inflated to the size of the siamang's head, allowing it to make loud, resonating calls or songs.