Abstract: An improved method and apparatus for forming cellular core structures from corrugated fibreboard utilizing flat sheets of single wall corrugated board of rectangular configuration. The sheets are fed end one at a time in the direction of the board flutes into a rotary die cutting station to form a parallel array of evenly spaced die cut lines extending perpendicular to the flute-feed direction from one side edge to the other side edge of each sheet. Each die cut line is defined by an alternating series of very long die cuts and very short die cuts individually spaced from one another along the die cut line by very short uncut sheet material connecting portions to thereby form a plurality of strips of sheet material intermittently integrally joined by the uncut sheet material connecting portions. An array of crease lines extend parallel to the flute-feed direction and are arranged in pairs of spaced apart in the direction of the die cut lines and closely flanking each of the uncut connecting portions.