The most common percolation model is to take a regular lattice, like a square lattice, and make it into a random network by randomly "occupying" sites (vertices) or bonds (edges) with a statistically independent probability "p". At a critical threshold "pc", large clusters and long-range connectivity first appears, and this is called the percolation threshold. Depending on the method for obtaining the random network, one distinguishes between the site percolation threshold and the bond percolation threshold. More general systems have several probabilities "p"1, "p"2, etc., and the transition is characterized by a "critical surface" or "manifold". One can also consider continuum systems, such as overlapping disks and spheres placed randomly, or the negative space ("Swiss-cheese" models).