A folding carpenter's level is disclosed which has two level segments, each of which has a smooth flat top and bottom surface, that are hingeably attached at a distal tip by means of a recessed, flat hinging means. The two level segments can be rotated laterally in the plane of the level so as to be oriented with respect to each other at angles of from 0 to 180 degrees, thus effectively doubling the length of the level. At least one of the level segments has one or more tubular spirit vials attached to the segment for measuring the levelness of the top surface with respect to horizontal. The level segments usually have additional tubular spirit vials oriented vertically to allow the tool to be used to measure plumb. Such tubular spirit vials are generally located in apertures so that the spirit bubble in the vials can be seen from the front or the back of the folding level. The folding level can beneficially include other recessed features, such as a fastening means to hold the segments together in a folded configuration, and a locking means to hold the segments secure when the segments are rotated 180 degrees with respect to each other so that the tops of the segments form a smooth flat surface with a straight edge.