Midline is a marking that spans across the middle section of a dinosaur.  This marking can have a hard, feathered or gradient edge.

This marking can be lighter or darker than the base color.
This marking can create stripes or blotches/spots, but only if it is near a larger area of color and appears to be coming off of it, not a seperate shape on its own. [See bottom example with feathered edge]  It cannot be made to mimic other markings.
This marking must be one solid color.