A flat-blend zone is a short stretch of bottom — typically under the back foot, around the wide point — where the board transitions out of one bottom contour and into another without a hard edge. Instead of ending the single concave abruptly and starting the double concave at the same line, the bottom flattens for a few inches and lets the two contours hand off smoothly.

What it does on a surfboard

The flat zone smooths the transition feel under your back foot. You don't feel a "click" between contours; the board reads as one continuous bottom even though you're moving between two different shapes. It also gives the shaper room to tune exactly how much each contour does — push the zone forward and the back third gets more double; push it back and the front 2/3 gets more single.

It's a Lundquist-favored detail that keeps the line feeling lively without sacrificing the drive that the back-third contour provides.

What to look for

Phrases like "flat-blend zone" or "flat transition" on a model page indicate that the shaper has deliberately built in this smoothing detail rather than running concave-to-concave directly.