For the legal system of canons, see Canon law and Canon law (Catholic Church). In Catholic canon law, a canon is a certain rule or norm of conduct or belief prescribed by the Catholic Church. The word "canon" comes from the Greek kanon, which in its original usage denoted a straight rod that was later the instrument used by architects and artificers as a measuring stick for making straight lines. Kanon eventually came to mean a rule or norm, so that when the first ecumentical council—Nicaea I—was held in 325, kanon started to obtain the restricted juridical denotation of a law promulgated by a synod or ecumenical council, as well as that of an individual bishop.