Before you read any further, think a while about whether you say or write log in and log out, or log on and log off, and whether the websites you usually use use those (or anything else).
Recently, I’ve been thinking about phrasal verbs, and this morning, before my students arrived and while were doing their fortnightly test, I scrolled through one online list of phrasal verbs. My eye was caught by ‘log in (or on)’ and ‘log out (or off)’. I couldn’t decide what I think/say/write of those two choices, except that I would pair in and out, or on and off, rather than in and off, or on and out.
I checked Google Ngrams, and after the students finished their test I talked to them about the way English is changing and how there are sometimes choices. (Part of what got me thinking about this was that one of the topics this week was money and shopping, and we had prepositional verbs like lend to, borrow from and pay in/by cash. The textbook gave pay in cash, and Google Ngrams shows that pay in cash is indeed more common than pay by cash, but the latter is certainly a valid option. The most used phrase is actually pay cash, which the textbook didn’t mention.)
I showed them this Ngram, which shows that ‘log on/in/off/out’ all increased in use from about 1980, which is not surprising, given the growth of computerisation from about then. Log in/out was the early leader, though by not much, until the mid-1990s, when log on/off took off and log on/off leveled off or declined.
One student noticed that the Ngram showed the years 1800 to 2000. I explained that those were the default years, then typed in 2008 (the last year of the Ngrams dataset), expecting that the results would be pretty much the same. To my surprise, we saw this: while log in/out continued to decline, log on/off plummeted, to the point where log out was more common than log off, and log on was only just remaining ahead of log in.
I suspect that around 2000, people stopped having to explain to other people about logging on/in/off/out; they could assume that as basic computer knowledge. I also suspect that the decline has accelerated since 2008. I am always logged in/on to various websites, especially on my home computer, but log off/out of websites at work, where other people might use my computer.
The first website I checked was Outlook email, which rather unhelpfully uses sign in and sign out.