As the ritsuryo system (a system of centralized government based on a set of codes) collapsed during the early to mid-Heian period, the system of taxation called so (tax) evolved into the nengu; the name nengu (annual tribute), however, kept something from its precedent, "koso" (the tribute-tax).
律令制における租が、平安初期～中期に律令制が崩壊・形骸化したことにともなって、年貢へと変質したが、貢租（こうそ）という別称に名残が残っている。