A cup forming sidewall blank includes a piece of flexible sheet material having an upper edge, a lower edge, and a pair of opposite side edges with an area at or adjacent the lower edge of the sidewall blank being provided with relief formations which extend generally upwardly and perpendicularly relative to the lower edge. The relief formations preferably take the form of score lines, slits, notches or other suitable constructions which are generally spaced from one another and are of equal length relative to the sidewall blank lower edge.