There is clearly a difference between those clubs with an 'old' or 'principal' 18, plus a relief 9 (as at the Gog Magog GC before the opening of the Wandlebury Course), and those with a nominally balanced set of three nines. In the British Isles, I guess that (maybe) Portmarnock and certainly St George's Hill and Prince's would be the best examples of the latter. I have only ever spent one day at lovely SGH, but recall that we were able to play all 27 in the course of our 36 hole day. At Prince's, on a windy spring Sunday afternoon, we could do pretty much what we wanted, but I guess that is not the norm. I suspect that in practice there is an established members' preference for the 'two best or most enjoyable' nines to go together, at least seven times out of ten?