Answer:

In a typical Flat, wave-c is normally a little longer or a little shorter than wave-a. As a result, it is unacceptable to allow wave-c to be more than 161.8% of wave-a in any standard Flat.

The one characteristic that distinguishes a normal Flat from an elongated Flat is the size of wave-c in relation to wave-a. As wave-c reaches 161.8% of wave-a, it moves into "elongated" territory. The maximum acceptable length of wave-c in an elongated Flat is around 261.8%. Beyond that, some other pattern is probably forming.

Elongated Flats should only be allowed as a single segment of a larger Triangle, Diametric, Symmetrical or one phase of a Complex Correction. Lastly, an elongated c-wave can be a Terminal, but it must be the last leg of a larger formation.