Topology can be used to abstract the inherent connectivity of objects while ignoring their detailed form. For example, the figures above illustrate the connectivity of a number of topologically distinct surfaces. In these figures, parallel edges drawn in solid join one another with the orientation indicated with arrows, so corners labeled with the same letter correspond to the same point, and dashed lines show edges that remain free (Gardner 1971, pp. 15-17; Gray 1997, pp. 322-324). The above figures correspond to the disk (plane), Klein bottle, Möbius strip, real projective plane, sphere, torus, and tube. The labels are often omitted in such diagrams since they are implied by connection of parallel lines with the orientations indicated by the arrows. The "objects" of topology are often formally defined as topological spaces. If two objects have the same topological properties, they are said to be homeomorphic (although, strictly speaking, properties that are not destroyed by stretching and distorting an object are really properties preserved by isotopy, not homeomorphism; isotopy has to do with distorting embedded objects, while homeomorphism is intrinsic). Around 1900, Poincaré formulated a measure of an object's topology, called homotopy (Collins 2004). In particular, two mathematical objects are said to be homotopic if one can be continuously deformed into the other. Topology can be divided into algebraic topology (which includes combinatorial topology), differential topology, and low-dimensional topology. The low-level language of topology, which is not really considered a separate "branch" of topology, is known as point-set topology.