It follows that the homology group "Hk"("S") is nonzero exactly when there are "k"-cycles on "S" which are not boundaries. In a sense, this means that there are "k"-dimensional holes in the complex. For example, consider the complex "S" obtained by gluing two triangles (with no interior) along one edge, shown in the image. The edges of each triangle can be oriented so as to form a cycle. These two cycles are by construction not boundaries (since every 2-chain is zero). One can compute that the homology group "H"1("S") is isomorphic to Z2, with a basis given by the two cycles mentioned. This makes precise the informal idea that "S" has two "1-dimensional holes".