Altogether as one word means completely, or in total — that's altogether different, £40 altogether. All together as two words means everyone or everything in one place or acting at once — they sang all together. If you can move "all" elsewhere in the sentence, use two words.
The core difference
An adverb meaning completely, and a phrase meaning as a group.
- altogether — completely, in total: altogether wrong, ten altogether.
- all together — as a group: we were all together at last.
The moving test
Try moving all. "We were all together" → "we were together, all of us" still works, so it is two words. "That is altogether wrong" cannot be split that way, so it is one word. This test works because in the two-word form, all is a separate quantifier.
The idiom
In the altogether is a dated euphemism for naked. It takes the one-word form and is worth knowing purely so it is not mistaken for a typo.
Frequently asked questions
Which means completely?
Altogether, as one word.
What is the test?
Try moving "all" elsewhere in the sentence. If it still works, you need two words.
What does "in the altogether" mean?
A dated euphemism for naked, using the one-word form.