Abstract: A reusable and adjustable arch-forming assembly employing a flexible sheet for placing construction materials thereon for the purpose of constructing an arch. The assembly has a center pole with a vertex support and two cross beams mounted by their proximal ends in slidable members engaged around the center pole. Each cross beam has an arch end support at its distal end. An auxiliary support mechanism aids in setting the inclination of the cross beams with respect to the center pole and provides mechanical support for the cross beams at the set inclination. The arch is defined by a curvature assumed by the flexible sheet between the vertex support and the two end support mechanisms. Supplementary arrangements of the assembly permit one to prepare any desired arch structure by adjusting the curvature of the flexible sheet.