An altar is a raised table used for worship or sacrifice: "They exchanged vows at the altar." To alter is to change something: "We altered the seating plan at the last minute." One is a thing you can stand in front of; the other is something you do.
The core difference
Altar is only ever a noun, and always a physical structure in a religious setting — a church altar, a stone altar, a household shrine. Alter is only ever a verb, meaning to make different. There is no sentence in which either can do the other's job.
Using altar
- Candles were lit at either end of the altar.
- She left him standing at the altar.
It appears in a small set of fixed compounds: altar boy, altar cloth, altarpiece, high altar. Figuratively, people sacrifice things "on the altar of" something — "jobs sacrificed on the altar of efficiency" — which is the one abstract use, and it still imagines a real altar.
Using alter
- Nothing you say now will alter the outcome.
- The dress needs altering before the wedding.
Its relatives all keep the -er: alteration, alterable, unaltered, alter ego. A tailor does alterations, never altarations.
The test
Swap in the word change. If the sentence still makes sense, you want alter — "nothing will change the outcome" works, so alter is right. If instead you can put in table and the sentence still stands, you want altar.
The memory trick
Alter ends in -er like changer and like the words in its own family: alteration, alternative. Altar ends in -ar like pillar and cellar, and like them it is a thing you could point at in a building.
Where it goes wrong
The classic error appears in wedding writing, where both words are in play at once and both are plausible: "we had to altar the ceremony" and "waiting at the alter" are both common. Since each is a real word, nothing will flag them.
Watch out too for alter ego — a second, contrasting side of someone's personality. It is two words, no hyphen, and never altar. And in worship contexts, note that the person assisting is an altar server or altar boy, while a person who changes something is simply the one who alters it; there is no noun "alterer" in ordinary use, so rephrase rather than reach for it.
Frequently Asked Questions
Is it "left at the altar" or "left at the alter"?
Altar. The phrase refers to the literal table at the front of a church, where the abandoned partner is waiting.
Is it "alter ego" or "altar ego"?
Alter ego — from the sense of a changed or other self. It has nothing to do with worship.
How do I remember which is which?
Alter belongs to the change family — alteration, alternative — and they all use -er. Altar ends in -ar like pillar and cellar, both physical parts of a building.