before the spanish conquest, the inca empire (1476-1534 AD) spanned farther than any on earth at the time – from argentina to colombia. but there is no evidence of a written language to communicate across these vast distances. instead, the inca are survived by a unique language of knots called quipu that was used to record census and taxes. the ingenious system is based on mathematical operations in base 10 encoded through three kinds of knots in a branching structure. quipu are so complex that some were used to encode legends and language instruction; what’s more they are highly portable and could be wrapped around the waist by runners traversing the empire on suspension bridges across the andes.