Quipu - ancient mnemonic and accounting system used by Incas and their predecessors in Andes. It is a complex system of joint strings and nods. Quipu was used for double-entry bookkeeping for the first time in human history. Famous Spanish chronicler (historian) (José de Acosta) wrote that "Whole Incan Empire was managed using Quipu, and nobody could avoid those who were doing calculations with the help of nods".