A set of plate segments for refining comminuted cellulosic material including: a first plate segment and a second plate segment, wherein the first plate segment and the second plate segment each have a refining side configured to oppose the refining side on the other segment, and each of said refining sides includes a refining zone having bars and grooves between adjacent ones of the bars, wherein distance between the grooves in the first plate segment and the grooves in the second plate segment is substantially constant along an arc through the refining zone while the set is mounted to discs in a refiner.