The normal order of a pc set is an arrangement such that the smallest interval possible exists between the first and last pcs in the set. To determine the normal order of a pc set, study the following process:

Given the notes: Ab, F, Bb, E

1. arrange them in ascending order and 
   add the lowest pitch on top:          Ab, Bb, E, F,  (Ab)

2. identify the interval between each
   successive pc:                        Ab, Bb, E, F,  (Ab)
                                            2   6  1   3

3. rearrange the notes beginning on the
   higher note of the largest interval:  E,  F, Ab, Bb

The normal for this pc set is {E,F,Ab,Bb}.

It is possible for there to be more than one largest interval between adjacent pcs. For example, the pc set {D,E,F#,A,C} has the largest interval of a m3 (ic3) in two different places (between F# and A, and A and C). Both {A,C,D,E,F#} and {C,D,E,F#,A} would be possible normal orders for this pc set, but one of them is the Best Normal Order. When you have a choice, the best normal order is the one where the smallest intervals are grouped to the left. In this case, that means {C,D,E,F#,A} would be the best normal order}.