ShapeA double bottom looks like the letter “W”.  Generally the second bottom should clearly undercut the first bottom
LengthMinimum 7 weeks to be valid (though it can take several months to form).
Depth: It usually features a more severe correction but no more than a 50% drop from it’s peak.
Pivot: The middle peak in that W-like structure.  That peak should also be in the upper half of the base.