A zigzag is a three-legged correction built 5-3-5: wave A divides into five subwaves, B into three, C into five again. It is the simplest and usually the deepest of the corrections.
What is visible on the chart
A sharp move against the trend, a short pullback, a second sharp move the same way. From the outside a zigzag looks like "two legs down with a breather" — the most recognisable of the corrective forms.
The key to identifying it is the internal build of the first leg. If wave A divides into five, it is a zigzag; into three, a flat. The depth of the B pullback does not affect the form, only the kind of flat.
Why C usually travels past A
In a typical zigzag wave C goes beyond the end of wave A: the counter-trend move does not run out of breath but resumes with new force.
When it does not — C fails to reach the end of A — the zigzag is called truncated. The form stays admissible: there is no hard rule saying "C must exceed A".
The reverse also happens: C much longer than A. Such a zigzag is called extended, and it often turns out to be part of a larger construction rather than a correction of its own.
How it differs from an impulse
This is the main confusion and it deserves attention. An impulse has five legs, a zigzag three. But inside a zigzag both A and C themselves divide into five, so at a fine scale a zigzag is easily taken for an impulse with a correction.
Scale of viewing tells them apart. Step down a degree and you see the fives inside the legs. Step up a degree and you see that the whole construction is three legs running against the containing move.
This is exactly why a wave number without a degree is meaningless.
Where else it appears
A zigzag is the workhorse of markup: it is wave A or C inside a larger correction, a leg inside a WXY combination, and the most common form for waves 2 and 4 inside an impulse.
Hence a practical note: spotting a zigzag is not enough — you have to say what it is inside something larger.
A common mistake
Declaring a zigzag finished on the strength of two legs and a breather. The third leg exists only once C has ended, and while C is running, "zigzag" is a hypothesis rather than a fact.