Already as one word refers to time — it means by now or sooner than expected: they've already left. All ready as two words means completely prepared: we're all ready to go. Time takes one word; readiness takes two.
The core difference
An adverb of time and a phrase about preparation.
- already — by this time: I've already eaten, is it noon already?
- all ready — fully prepared: the bags are all ready.
The test
Remove all. "The bags are ready" still works, so it was two words. "I have ready eaten" does not work, so it was already. The same test handles altogether/all together, which is the sibling pair.
The pattern
English lost an L when these compounds formed: all + ready → already, all + together → altogether, all + most → almost, all + so → also. Spotting the dropped L identifies the one-word form every time.
Frequently asked questions
Which one means prepared?
All ready, as two words.
What is the test?
Remove "all". If the sentence still works, you needed two words.
Why does already have one L?
English dropped an L when these compounds formed — the same happened in almost, also and altogether.