A diagonal is an impulse in which waves 1 and 4 are allowed to overlap. Five legs, as in an ordinary impulse, but the boundaries converge and the ban on the fourth wave entering the first's territory is lifted.
How it differs from an impulse
The only substantive difference is the suspended overlap rule. Everything else stays: five waves, numbering 1 to 5, alternating directions.
But a suspended rule does not mean no rules. In its place: wave 4 does not go beyond the end of wave 2. Break that and the markup is discarded exactly as it would be for an impulse violation.
A leading diagonal carries an extra condition: the pullback must not go beyond the start of the diagonal itself.
How it is identified
Not by a wedge shape to the eye, but by the geometry of the legs: an overlap of waves 1 and 4 while the five-wave structure holds and the boundaries converge.
Those converging boundaries — lines through the ends of the legs — are what set where the missing wave can end. The closer the meeting point, the less room is left: in a diagonal the space for completion narrows by itself, unlike in an ordinary impulse.
Where it occurs
A diagonal takes a position where an impulse no longer fits: at the end of a move, as the fifth wave or as wave C, or at the start — as the first wave or as wave A.
Hence the two names, ending and leading diagonal. It is one and the same form in different positions, not two different objects with different rules.
Diagonals and truncation
Both come up in the same context — at the end of a move — and both mean the fifth wave behaved unusually. But they are different things: a diagonal is about overlapping legs, truncation is about the fifth failing to reach the end of the third. There is a separate write-up on truncation.
What a diagonal does not mean
A diagonal appearing does not mean the move is about to end. It is a statement about the structure of a stretch already travelled: the legs assembled with an overlap, so impulse rules would have discarded this markup while diagonal rules permit it.