Common gaps appear in quiet, thinly traded conditions or simply because the overnight session drifted. They occur inside consolidation rather than at boundaries, and volume around them is unremarkable.
They are the gaps that make gap-fill statistics look good, because they close so reliably. They are also the least worth trading: small gaps mean small targets, and after costs the edge tends to disappear.
The useful discipline is classification before action. If a gap is small, newsless and inside a range, treat it as noise; if it is large, at a boundary and on volume, treat it as information. Applying one gap rule to every gap is how traders lose money on the tails.
Related: gap-fill, breakaway-gap, gap, consolidation, session-breaks