Purlin is a Lovely Word

So if there's a rough 3 1/2 foot increment that's supposed to organize everything, and the mullions in the curtainwall double up at every third bay, do you let the grid vary to keep the panel size even, or do you maintain the rigor of the system, let it define the centers, and just use two panel sizes? It's an easy question until you remember that the 3 1/2 foot lines organize the spacing of the purlins at the roof as well, and there's no reason for them to vary on the thirds. A nonexistent system can become many things, a real one is limited by the circumstances that brought it into being. Abstraction wins.