A thing that has struck me from time to time is the surprising number of things that come in varieties labelled as ‘1’ and ‘3’, or ‘1st’ and ‘3rd’, missing out 2.

I tend to think of this as ‘Trinity Boat Club Syndrome’, since my standard example case is the 1st and 3rd Trinity Boat Club. (I've always faintly wondered what happened to the 2nd, and had rather hoped it would have been disbanded in mysterious circumstances after the inadequately explained sinking of a John's boat, or possibly an Oxford one; I was recently disappointed to find out that their website actually answers the question and it's nothing so exciting.)

There are several other common ones. You hear a lot about the Third World and the First World, but ‘Second World’ is not a widely used phrase by comparison. Fiction tends to appear in single novels or trilogies, with relatively few two-volume works in between. And both books and computer games are often described as first-person or third-person, but very few of either are second-person.

More domain-specific examples include the fact that in technical drawing, orthographic projections come in first angle and third angle, but never second, and that in violin-playing it's common to use first position and third position, but second position (while a perfectly meaningful concept) comes up rather less often.

So: what other examples of this haven't I thought of, and what can we do to rehabilitate the poor left-out number 2?