A hierarchy of ridge segmentation defined by axial discontinuities for (A) fast- and (B) slow-spreading ridges. S1, 2, 3, 4 are ridge segments of order 1, 2, 3, 4 and D1, 2, 3, 4 are ridge axis discontinuities of order 1, 2, 3, 4. At both fast- and slow-spreading centers, first-order discontinuities are transform faults. Examples of second-order discontinuities are overlapping spreading centers (OSCs) on fast-spreading ridges and oblique shear zones on slow-spreading ridges. Third-order discontinuities are small OSCs on fast-spreading ridges and inter-volcano gaps on slow-spreading ridges. Fourth-order discontinuities are deviations from axial linearity (devals) resulting in slight bends or lateral offsets of the axis of less than 1 km on fast-spreading ridges and are intra-volcano gaps on slow-spreading ridges. This four-tiered hierarchy of segmentation may really be a continuum. It has been established, for example, that fourth-order segments and discontinuities can grow to become third-, second-, and even first-order discontinuities and vice versa at both slow- and fast-spreading centers.