It is very common for "n" = "p" + "q" to be another natural index in the spectral sequence. "n" runs diagonally, northwest to southeast, across each sheet. In the homological case, the differentials have bidegree (−"r", "r" − 1), so they decrease "n" by one. In the cohomological case, "n" is increased by one. When "r" is zero, the differential moves objects one space down or up. This is similar to the differential on a chain complex. When "r" is one, the differential moves objects one space to the left or right. When "r" is two, the differential moves objects just like a knight's move in chess. For higher "r", the differential acts like a generalized knight's move.