Dzongkha, the national language of Bhutan, has two numeral systems, one vigesimal (base 20), and a modern decimal system. The vigesimal system remains in robust use. Ten is an auxiliary base: the -teens are formed with ten and the numerals 1–9. Ex. cu_ci
Vigesimal Numerals Spelling Romanisation Hindu-Arabic Tibetan 1 gcig 2 gnyis 3 gsum 4 bzhi 5 lnga 6 drug 7 bdun 8 brgyad 9 bgu 10 bcu tham* 11 bcu gcig 12 bcu gnyis 13 bcu gsum 14 bcu bzhi 15 bco lnga 16 bcu drug 17 bcu bdun 18 bco brgyad 19 bcu dgu 20 khal gcig *When it appears on its own, ‘ten’ is usually said bcu tham ‘a full ten’. In combinations it is simply bcu.
Multiples of 20 are formed from khal. Intermediate multiples of ten are formed with phyed 'half to':
30 khal phyed gnyis (a half to two score) 40 khal gnyis (two score) 50 khal phyed gsum (a half to three score) 100 khal lnga (five score) 200 khal bcu tham (ten score) 300 khal bco lnga (fifteen score) 400 (20²) is the next unit: 400, 800, etc. Higher powers are 8000 (20³) ('a ɡreat score') and 160,000 (20⁴).
Decimal The decimal system is the same up to 19. Then decades, however, are formed as unit–ten, as in Chinese, and the hundreds similarly. 20 is reported to be , the same as vigesimal numeral 400; this may be lexical interference for the expected . (In any case, there is no ambiguity, because as 400 it is obligatorily 'one 400'.) Several of the decades have an epenthetic , perhaps by analogy with 18 and 19, where the presumably reflects a historical 'ten':
30, 40, 50, or 100 (a 'full hundred' or 'one hundred'), 200, 300, 400, etc.
References Mazaudon & Lacito, 2002, "Les principes de construction du nombre dans les langues tibeto-birmanes", in François, ed. La Pluralité, p. 6 ff