The primary crack in flake-making actually progresses through the stone in minute ‘tongues’ emanating from the fracture front.  These tongues join up as the fracture front progresses.  Towards the centre of the front, where the tensile stress is closest to a right angles to the fracture surface, the space between the tongues is microscopic and the fracture plane appears continuous and smooth to the naked eye.  At the edges of the crack, however, the tensile stress is more oblique, and the tongues do not join up as cleanly because of greater displacement between them.

Sometimes the tongues travel short distances parallel to each other after the main fracture front passes.  When these displaced tongues are joined, a needle-like sliver is either broken away as knapping dust, or part of it can remain attached to the scar on the core or the ventral surface of the flake.  We see these partly-attached tongues as hackles.  They are usually less than a millimetre long (as measured in the direction of propagation), but they can be many millimetres wide.  An eraillure is a special type of hackle that propagates across the bulb of force on conchoidally-initiated flakes.