equ

The root equ comes from the Latin adjective aequus, which meant "level, even, flat" — and, by a short metaphorical step, "equal, fair, calm." Picture a flat plain or a still surface of water: nothing sticks up higher than anything else, no part outweighs another. That single image of evenness is the seed of the whole family.

The most literal branch is mathematical and quantitative. equal is simply "even, the same amount"; equality is the state of being even; equate is to make two things even; an equation is a written statement that the two sides balance exactly. Here aequus keeps its plainest sense: same size, same value, nothing tipping the scale.

From "even" the meaning slides naturally into "fair." If you treat people evenly, you treat them justly. So equity is fairness (and, by a later financial twist, the "fair" leftover value you own in a house or company once debts are paid); equitable means fair to all sides; egalitarian (through French égal) describes the belief that everyone should be on the same level. Negate the fairness and you get the dark members: inequity (in- + aequus) is plain unfairness, and iniquity — same Latin pieces, in-aequitas, "un-evenness" — drifted all the way to "gross wickedness, sin." Unfairness, taken to its extreme, became evil itself; iniquitous is its adjective.

A third branch is "matching a standard." adequate is ad- (to) + aequus: "made equal to" what is required — hence just enough, up to the mark. equivalent (equ + val, worth) is "of equal worth."

Then there is the calm sense of aequus — an even mind, a level temper. equanimity (equ + anim, mind) is a mind that stays level under pressure; equanimous and equable describe that same unruffled steadiness.

Finally, two words apply "even/balance" to the physical world. The equator is the line that equalizes the globe, splitting it into two even halves. equilibrium (equ + libra, balance/scales) is the point where opposing forces are perfectly even and nothing moves. And equivocate (equ + voc, voice) is to give two meanings "equal voice" at once — to speak so ambiguously you never have to commit.

One caution: a different Latin word, equus ("horse"), looks identical but is unrelated. Equestrian and equine belong to the horse, not to fairness. If a word is about being level, equal, fair, or calm, it is the aequus family; if it is about horses, it is not.