Gibbons exhibit a number of primitive characteristics which they share with the Cercopithecoidea, whereas in some other characteristics they are the most derived of all the modern Hominoidea (for instance in their limb proportions).

• Body size: dwarf gibbons (genus Hylobates) 5-7 kg, hoolocks (Hoolock) and crested gibbons (Nomascus) 7-10 kg, siamangs (Symphalangus) 10-12 kg. There is virtually no sexual dimorphism in either the canine teeth or body size.
• relatively simple molar teeth with low, rounded cusps, sectorial front teeth and long, daggerlike canines in both sexes (Figure 4.1)
• short snout, large orbits, relatively wide inter-orbital distance, globular braincase, shallow mandibula with broad ascending ramus (Figure 4.1)

Figure 4.1. Skulls of various recent members of the Hominoidea (Hylobates: unknown sex, Pongo and Gorilla: males, Pan and Homo: females). Note the pronounced supra-structures (nuchal- and sagittal crests) in the male skulls, especially in Gorilla. All skulls were brought to approximately the same size of the cranial vault (Hylobates after Schultz, 1944, p. 88; Pongo after Schultz, 1941, p. 99; Gorilla und Homo after Schultz, 1972, pp. 126 and 127; Pan after Schultz, 1940, p. 52).

• Compared to all other primates, gibbons have unusually specialised extremities. Relative to their body size, gibbons have the longest arms, but also very long legs.
• The phalanges of hands and feet are long and curved (see Figure 1.5). The first digit of the hands and feet is long and extremely separated from the from the palmar or plantar area (seeFigure 4.2).

Figure 4.2. Feet (left) and hands (right) of Hominoidea members (after Biegert, 1963, pp. 3/261, 268, 280, and Biegert, 1973, p. 171). Note the extension of the phalanges and the basal separation of first digit in the gibbon (Hylobates), as well as the reduction of the first digit in the orangutan (Pongo).

• The scientific names Symphalangus and syndactylus both mean 'fingers that have grown together'. This is because in the feet of the siamang, the second and third digit are actually joined by connective tissue at the base. This condition is known as syndactyly. It occasionally occurs in other gibbon species, as well, but is quite rare and the connection is usually less extensive.
• Gibbons are the only apes, that consistently exhibit ischial callosities. In addition, the females have noticeable sexual swellings, during which the Labiae majorae undergo cyclic changes in their colour and form.