A double concave is two parallel hollow channels carved into the back third of the bottom, separated by a low ridge or stringer-side spine. Water that flowed through the single concave up front splits into the two channels in the tail, accelerates further, and exits through the fins.

What it does on a surfboard

Double concave in the back third does two things at once: it adds drive when the board is on rail (the channels guide water past the rail-side fins) and it releases water cleanly off the tail so the board doesn't bog when you weight the back foot. The combination of single → double is the most common modern HPSB / step-up bottom contour.

The depth of the double matters. A "soft double" or "slight double" is a gentle transition that keeps the board lively across all rail angles. A deeper double pushes more drive but can feel locked-in.

What to look for

Phrases like "soft double concave," "slight double concave," or "vee-blended double" all describe variations on the same back-third contour family. Lundquist boards lean toward soft and slight doubles — the line favors a livelier feel under foot over maximum drive.