Usages,1 such as those of a trade,2 may be resorted to to show the special meanings of words. Thus evidence of local usage as to the meaning of "cord" in a sale of cedar posts, or of a trade usage as to "subject to strikes" in a contract for the sale of coal,3 is admissible, or as to "on approval" in the diamond trade.4 If the meaning of a written contract is clear, a trade usage cannot change the meaning of the words, or add incidents so as to contradict the meaning.5 Thus a contract with a broker for the sale of certain articles, "seller paying brokerage at ten cents per ton," cannot be contradicted to cut down the broker's recovery by showing a usage to pay commissions only on the amount delivered.6 No usage can be invoked to change rules of law. Thus a usage among brokers that stock certificates are negotiable is invalid.7