For two years the diary promised times that no room existed for. Booking the chair fixed it in a week.
For the first two years the diary here worked the way most clinics work: an appointment was a time with a dentist. It went wrong every single week, and always in the same way.
Two dentists share three chairs across a session. That is not understaffing, it is how dentistry works: a filling that is setting does not need anybody standing over it, an injection needs ten minutes to take effect, and during both of those the dentist is usefully in another chair. So a session has more dentist time in it than chair time, and a diary that books dentists will cheerfully sell a nine o'clock to three people when there are three chairs and one of them is the hygienist's.
So we turned it round. The thing the booking form holds is a chair. One and two in the front room, three in the quiet room at the back, four for the hygienist. Your confirmation email names the chair. If chairs one to three are free at nine and chair four is booked out for a fortnight, the form tells you exactly that, and the scale and clean you wanted really is two weeks away.
It makes the website look less available than the competition, which we did think about. It also means nobody has been told at the desk that there has been a mix up since the day we changed it.
The other thing it gave us for nothing was the quiet room. Once a chair is a thing you can ask for by name, offering the room at the back to somebody who is dreading the appointment becomes a checkbox rather than a favour.