Where did this one on one thing come from? A one on one is a boxing match or a schoolyard fight, not an open chat.
8:2:1 is a ratio of cocktail ingredients. 16:9 is an aspect ratio. 1:7 is a steep hill to climb. To, by, and in respectively. The latter two would be weird choices but at least they’d be consistent with the syntax of x:y. “on” though?! Where did that come from?
Don’t say “Japanese poetry” :)
Now one to one implies an equal relationship and especially in our field it feels weird imo to call them one to ones. Typical, you aren't equal levels in a 1:1 meeting. So maybe the one on one caught on because of the subtlety of one being on top of or above the other.
Face to face sounds too literal and agressive.
1:1 is an easy clear way to represent one on one. Although, we are forcing a double meaning on it. But hey that's the great thing about language. It's ever evolving.
Personally, in meeting titles I write PersonA/PersonB but I do type 1:1 over one on one when referring to the meeting type.
The whole point of a 1:1 is an opportunity to see your direct as a person and not as a direct. It puts you on equal playing ground for the moment, and as a manager, it is your responsibility to make sure your direct feels that way. When they trust you, they'll tell you how they really feel. That means being open about what they want out of their career, when they are thinking about leaving, when there's an uncomfortable problem on the team. That is what 1:1 is for, and that's why I always refer to them as a one-to-one and not a one-on-one.