Quid pro quo is a Latin phrase used in English to mean an exchange of goods or services, in which one transfer is contingent upon the other; "a favor for a favor". Phrases with similar meanings include: "give and take", "tit for tat", "you scratch my back, and I'll scratch yours", and "one hand washes the other". Other languages use other phrases for the same purpose.
Antichristus, a woodcut by Lucas Cranach the Elder of the pope using the temporal power to grant authority to a ruler contributing generously to the Catholic Church
Tit for tat is an English saying meaning "equivalent retaliation". It developed from "tip for tap", first recorded in 1558.
In Western business cultures, a handshake when meeting someone is a signal of initial cooperation.