quadru

The root quadru- (also spelled quadri-) comes from Latin quattuor, "four," and it is the number-four building block in a long line of precise, often technical words. The logic is wonderfully simple: take quadru-/quadri-, attach a word for a thing, and you've specified four of them. quadri- + later (side) → quadrilateral, a four-sided figure. quadru- + ped (foot) → quadruped, a four-footed animal. quadri- + angle → quadrangle, a four-angled shape (and, by extension, the four-sided courtyard at the center of many old colleges, the 'quad'). Other members fill out the family: a quadrant is a quarter of a circle (one of four parts), a quadruplet is one of four children born together, and to quadruple something is to multiply it by four. Latin counting prefixes travel in a set, and quadru- sits right in the sequence: uni- (one), bi- (two), tri- (three), quadri- (four), quint- (five). Notice how quadruped lines up with biped (two-footed) — same -ped root, just a different number bolted on the front. The everyday surprise of this family is quarter. It looks unrelated, but it descends from Latin quartus, 'fourth' (itself from quattuor), through Old French quartier. A quarter is one of four equal parts — of a dollar, of an hour, of a year (a financial quarter), of a city (the old quarter). It's the same 'four' DNA, just worn smooth by everyday use and a detour through French. So when you meet a quadru-/quadri- word, count to four: four sides, four feet, four angles, four parts. The number is always sitting right there at the front of the word, telling you exactly how many.