Abstract: A single blank is used to make an expandable envelope construction having four side edges which can expand as the envelope construction is filled to capacity. The blank is provided with horizontal and vertical score lines and fold lines which permit the sides and bottom of the construction to collapse inwardly while the pocket enlarges in thickness and decreases in height and width. The blank has side flaps with rounded comers at the ends which are overlaid by the back panel and closure panel in an unsecured manner to maintain the envelope in a sealed condition when expanded.