It's probably worth mentioning that linear spreads with an odd number of cards (3, 7, 9) lend themselves to the use of Elemental Dignities, which work best in triplets with a center focus or "principle" card and two modifiers, one before and one after. Its just a method for weighting or adjusting the potency of a card according to how "friendly" or "unfriendly" it is with its neighbors
Oh yes, and not only elemental dignities but also planetary and numerological dignities. (I'm reading Liz Hazel's book! did anyone notice ???)
Hmmm. I've been doing this all along without even realizing it. I don't think I use any spreads with even numbers of cards except the CC, which I don't use very often. I just naturally gravitated to this I guess ...