Triangles are usually formed in a position prior to the final wave in an impulse or a correction. For example, a triangle could be formed in a wave four in an impulse or wave B in a zigzag .

Also, this pattern occurs in final wave X in a double/triple zigzag or three patterns.

Wave two of an impulse can’t be a triangle.
Waves A, B and C are usually zigzags, double zigzags, triple zigzags (that’s rare), double and triple threes.
Waves D and E could be triangles themselves.