Given that runways are numbered according to compass heading, 0/36 being due north, 9 being due east, 18 being due south, and 27 being due west.
I've noticed that there's a system of assigning L, R, and C to three parallel runways.
However, in cases of four or more parallel runways (i.e. at DFW) there's a problem so one pair of runways is numbered one degree higher/lower.
Lately, though, I've noticed yet another way that this is handled -- the two longest runways are marked as parallels and the shorter is assigned a different number. For example, at PHL three runways are parallel (except for a 0.1-degree difference). The two longer runways are marked 9L/27R and 9R/27L, while the shorter runway is marked 8/26.
The proposed third runway at CLE is marked the same way as PHL's -- although it appears to be parallel to the 5/23 parallels, it is marked as 6/24.
Why are PHL's runways not marked 9R/27L, 9C/27C, and 9L/27R?
Can someone explain the logic of this to me?