(1) Impulses have 5 waves. Corrections have 3 waves. (2) Prechter says that corrections should "alternate" when looking at the "2" and "4" waves in the 5-wave sequence, i.e. "2" can be a 3-3-5(flat-correction) while you would expect "4" to be a 5-3-5(zig-zag) or a 3-3-3-3-3, something different...