(a) Body weight 5–7 kg; no throat sac; crown fur without crest - Hylobates klossii
(b) Body weight 6–10 kg; throat sac very small and visible only when inflated during certain vocalisations; crown fur stands straight up, with prolonged hairs forming a crest - Nomascus concolor (male)
(a) Cheek beard pale yellow or orange, standing out on sides as if "brushed" outwards, with the hairs spreading out from the corners of the mouth like a fan; fur on chest rusty brown, not black - Nomascus gabriellae (male)
(b) Cheek beard white, rarely pale yellow, not "brushed" outwards; fur on chest black - Nomascus leucogenys (male)
(a) White face ring relatively broad and usually complete; no light corona around crown of head; hands and feet white up to wrist and ankle, respectively; genital tuft not white - Hylobates lar
(b) White face ring relatively thin, broad only above eyes; light corona around crown of head; hands and feet white only distally of wrist and ankle, respectively; white genital tuft - Hylobates pileatus (male)
(a) White brow band; body weight 6–9 kg; no light cheeks; distinct goatee beard; long genital tuft (ca 7.5 cm) - Hoolock hoolock (male)
(b) White or grey brow band (often reduced in older females); body weight 4.5–7 kg; males often with light cheeks (grey or whitish), females often without these; no distinct goatee beard; males with distinct, but relatively small genital tuft (ca 5 cm) - Hylobates agilis
(a) Back and limbs pale yellow, yellow, orange or beige brown, not grey or dark brown; black cap usually set off sharply from surrounding lighter fur, with crown fur (including cap) standing straight up body weight 6–10 kg - 11
(b) Back and limbs pale yellow, yellow, orange or beige brown, but also grey or dark brown; black cap, if present, with crown hairs lying flat; body weight 4–9 kg - 13
(a) Fur on chest (and sometimes belly) black or dark brown, contrasting with light back - Nomascus concolor (female), but excluding N. sp. cf. nasutus hainanus or N. sp. cf. nasutus ssp. nov.
(a) Face ring usually yellowish (rarely white), often not contrastingly lighter than neck or incomplete; cheek fur usually standing out on sides, as if "brushed" outwards, with the hairs spreading out from the corners of the mouth like a fan - Nomascus gabriellae (female)
(b) Face ring usually white and distinctly lighter than neck, often thin, but usually complete; cheek fur not standing out on sides - Nomascus leucogenys (female) or N. sp. cf. nasutus hainanus or N. sp. cf. nasutus ssp. nov. (female)
(a) Crown fur lying flat and being light beige, contrastingly lighter than dark brown cheeks; body weight 6–9 kg; a thin stripe of white hair starts from cheeks and crossing the face below the eyes goes upwards across the nasal ridge; distinct light goatee beard - Hoolock hoolock (female)
(b) Crown fur variably lying flat or standing up, not contrastingly lighter than cheeks; body weight 4–8 kg; no stripe of white hair below eyes and across nasal ridge; with or without distinct light goatee beard - 14
(a) Cap and ventral area black, both sharply set off from surrounding light grey fur; ventral shield three-cornered, with lower tip reaching genital area; back and limbs light grey or cream; no light face ring; thin white brow band may be present but is usually lacking; long, white temple fringes hanging over black cheek region and ears - Hylobates pileatus (female)
(b) Cap and ventral area may or may not be darker than surrounding fur; if these areas are black, they are rarely set off sharply and the ventral area has no distinctly three-cornered pattern; back and limbs light or dark, of variable colouration; with or without light face ring; light brow band usually present; no long, white temple fringes hanging over black cheek region and ears - 15
(a) White hands and feet; fur colouration variable: pale yellow, yellow, beige brown, hazel, dark brown (or black, see also 8a), but not grey; usually no dark cap and no dark ventral area (if present, these areas are brown or dark brown, not blackish); face ring white and usually complete - Hylobates lar
(b) Hands and feet not white; fur colouration variable as 15a, but may also be grey; with or without dark (dark brown or blackish) cap and dark ventral area; face ring only partly white, usually incomplete, often reduced to brow band - 16
(a) Silvery grey fur colouration; only cap and ventral area may be contrastingly darker (dark-gray or black) in some animals; brow band broad and whitish, usually distinct and sharply set off, pointed laterally of the eyes; distinct, forward projecting, whitish goatee beard - Hylobates moloch
(b) Fur colouration variable; grey individuals are mouse grey rather than silvery grey; cap, underparts and limbs may be contrastingly darker (dark grey, blackish brown or black) than back; hands and toes contrastingly black in some animals; brow band variable in both colouration (whitish, light grey, light buff) and extent (broad, thin, absent); no distinct, forward projecting, whitish goatee beard - 17
(a) Fur colouration variable: mouse grey, grey brown, hazel, or dark brown, but not yellow; feet may be distinctly paler than legs in some animals (but not white as in H. lar); light brow band very variable in its extent, more or less distinct, but rarely missing; usually no distinctly light cheek patches; genital tuft of male very small (ca 2.5 cm) and usually blackish - Hylobates muelleri
(b) Fur colouration variable: pale yellow, yellow, beige brown, grey brown, nut-brown, dark brown (or almost black, see also 9b); feet not distinctly paler than legs; brow band usually white, but may be completely absent in older blackish females; males often with grey, whitish or pale brownish cheek patches (often joined under the chin), females often without these; males with distinct, but relatively small genital tuft (ca 5 cm) which may be of contrastingly light colour in some brown animals (intermediate phase), but not contrasting with ventral fur in others - Hylobates agilis
