When a contract consists of reciprocal promises to be simultaneously performed, no promisor need perform his promise unless the promisee is ready and willing to perform his reciprocal promise.
Illustrations
    (a) A and B contract that A shall deliver goods to B to be paid for by B on delivery.
    A need not deliver the goods, unless B is ready and willing to pay for the goods on delivery.
    B need not pay for the goods, unless A is ready and willing to deliver them on payment.
    (b) A and B contract that A shall deliver goods to B at a price to be paid by instalments, the first instalment to be paid on delivery.
    A need not deliver, unless B is ready and willing to pay the first instalment on delivery.
    B need not pay the first instalment, unless A is ready and willing to deliver the goods on payment of the first instalment.