One Latin phrase has come up over and over in the past year: quid pro quo. It defines the central question of President Donald Trump’s dealings with Ukraine – did he make military funding contingent on an investigation into Joe Biden and his family? This is a language column, so we won’t go into the politics. Instead let’s talk about the many words and phrases, like quid pro quo, that English has acquired from the multifarious Latin pronouns quis and qui.

These pronouns are often indistinguishable in Latin, and mean more or less the same thing: “who,” “what,” “which,” and “someone, something.” Quis is clearly visible in the Roman satirist Juvenal’s famous question, “Quis custodiet ipsos custodes?” or “Who will guard the guards themselves?” In English, this phrase is often used to highlight the dangers of corruption among the powerful. They watch us; who watches them?

But Latin nouns and pronouns decline, changing shape depending on the role they play in the sentence. Quid pro quo itself is just two different forms of quis, its neuter nominative form quid plus its ablative form quo. The phrase means “something for something.”  

Quomodo is “the manner, the means,” as in these lines from Henry Fielding’s classic “Tom Jones” (1749): “Northerton was desirous of departing ... and nothing remained for him but to contrive the Quomodo.” 

The feminine form of quo is qua, which gives us the sine qua non: the “without which, not,” or better translated, the “essential, indispensable.”

In the dative case, used to indicate the recipient of a thing or action, quis becomes cui. “Cui bono?” – the question asked in hundreds of courtrooms and detective novels – means “to whom for a benefit?” or, more elegantly, “who profits?”

The genitive plural contributed quorum (“of whom”), which means “the number ... of officers or members of a body that when duly assembled is legally competent to transact business.” 

In English, quid is related both to the deepest mysteries of being and to the glibbest trivialities. From the 17th century, quid has meant “that which a thing is,” its essence. When a character in a 19th-century play says, “My age has seen ... the quid of things,” he is talking about the wisdom he has gained as he has grown older. But quid also produced quiddity, which often means “a trifling point”; quibble, to make frivolous objections; and quip, a cutting or witty remark.  

When mathematicians finish a proof, they can employ another qui relative, quod erat demonstrandum – “(that) which was to be demonstrated.” This indicates they believe they have proved the problem satisfactorily. So, Q.E.D., or as my high school geometry teacher translated it, “quit, enough done.”