plain vs plane

The core difference

Plain is mostly about simplicity and clarity. Plane is mostly about flatness and, by a separate route, about flying. They are pronounced identically, so nothing but spelling separates them on the page.

Using plain

As an adjective, plain covers unadorned, straightforward and — of a person's appearance — ordinary.

• I'll have the plain yoghurt, not the flavoured one.
• Let me be plain: this cannot continue.
• The instructions are written in plain English.
• It was hidden in plain sight.

As a noun, a plain is a broad, flat, largely treeless stretch of country: the Great Plains, the plains of Hungary. This is the sense that makes people hesitate, because flatness feels as though it should belong to plane.

Using plane

Plane has several distinct senses, all spelled the same way:

• Aircraft: we caught the last plane out of Rome.
• Geometry: two lines in the same plane.
• Level or stage: the discussion moved onto a different plane.
• Tool: he planed the door until it shut cleanly.
• Tree: the London plane, common in city streets.

The test

Two quick checks. If you could substitute simple, clear or obvious, write plain. If the word means a flying machine, remember it is a clipping of aeroplane — the letters are already there, so keep the e.

For the awkward flat-land case, note that a geographical plain is a place you can stand in, and place words here take the i: plain, like Spain, mainly on the plain.

The mistakes to watch for

The most frequent error is "plane English" or "plane sight" — both need plain. Running the other way, "plain crash" appears surprisingly often in hurried writing.

Two idioms are worth pinning down. Plain sailing, meaning easy progress, is spelled with an i in British English; the American equivalent is usually smooth sailing. And plainclothes police wear ordinary clothes, nothing to do with aircraft.

Finally, plain as an adverb meaning simply — "that's just plain wrong" — is informal but perfectly established, and still takes the i.