help with word(s) on line?

It's pretty much finished evolving into one word, 'online', in both US and British English.

I've seen on-line and online ... pick one and stick to it throughout your text.

Online and offline seem to be winning the battle.

I note that both Apple and Microsoft's Style guides use online as one word, no hyphen, though off-line is hyphenated. They both go back to slightly before 1950, so I got no idea why one is hyphenated and the other not.

It might be that off-line is used less frequently, so there's less linguistic friction to wear down the hyphen, but I'm guessing.

As an American transplant to the UK, I've noticed that the process of a word going from being hyphenated to being one word is longer in the UK. For example, my Concise Oxford Dictionary (Tenth Edition, copyright/printing 1999--good grief; I need to invest in a new copy!) lists 'coordinate' as the primary spelling but includes 'co-ordinate' as an optional spelling; I don't remember ever seeing 'co-ordinate' used in a US publication, and I'm fast approaching 50.

Anyway, the same Concise Oxford lists 'online' as being unhyphenated and doesn't include the hyphenated version as an option.

Which doesn't mean you can't hyphenate it if you really want to, as long as you're consistent. But the process to 'online' seems to have been completed even in British English at least a decade ago.