Have you noticed the silent battle that’s being waged across the internet?
Sites and applications worldwide are deeply divided (sometimes even internally) between the compu-speak control "log in", its one-word cousin “login”, and its more human alternative "sign in."
Is there one right answer?
Nope. Both “Log In” and “Sign In” are acceptable options and can be found on highly trafficked American sites.
Neither are grammatically incorrect, but each have pros and cons.
Signing in is likely the more human-centered term, as it has an equivalent action in the physical world - the idea of signing in at a location or event. Logs, on the other hand, are something that we associate with computer systems.
That said, Sign In has a significant downside - it can be more easily confused with Sign Up. I’ve seen great confusion in usability testing when Sign In and Sign Up links were positioned alongside one another, as users often move so quickly through an interface that they entirely skip the second word of the phrase.
So, is "log in" one word, or two?
When it comes to Log In vs Login, two are used so interchangeably that many would argue that both are acceptable - but that’s just not true.
To be grammatically correct, “login” can either be a noun or an adjective, whereas “log” is used as a verb denoting an action. For example:
Noun: The login is too complicated for users to understand.
Adjective: You must enter your login credentials correctly to gain access into your online bank account.
Verb: Please log in to your online bank account at this time.
If you find this hard to remember, think of other parallel phrases - you don’t see Log Out, Sign Up, Sign In, or Sign Out presented as one word buttons or links, so why would you see Log In as one word?
So if you’re having an internal battle over which variation to use, think about it in the grammatical sense of the word: Users must enter their login credentials to log in, otherwise the login will confuse them.
Capitalization
So what about the capitalization of “log in” within a title or on the button on a website? According to Grammar Girl, there is technically no one right way to do this, but there is a wrong way. Once a style has been selected, stay consistent.
The question is really that of sentence casing vs title casing. To title casing, Associated Press style says that one should “capitalize the first word of the title, the last word of the title, and all ‘principal words’ … and all words longer than three letters.” If your site uses title casing elsewhere, both words of Log In should be capitalized when presented alone in buttons or links. The phrase should be presented as “Log in” if followed by additional copy - for example, “Log in to See Balance.”
A sentence-cased site wouldn’t capitalize the In, opting instead for a button that reads “Log in”.
The most important message to take away here: Remain consistent once you choose a style.