Sector numbering on a hard disk. A one to one interleave (1:1) is sequential: 0,1,2,3, etc. A 2:1 interleave staggers sectors every other one: 0,4,1,5,2,6,3,7.

In 1:1, after data in sector 1 is read, the disk controller must be fast enough to read sector 2, otherwise the beginning of sector 2 will pass the read/write head and must rotate around to come under the head again. If it isn't fast enough, a 2:1 or 3:1 interleave gives it time to read all sectors in a single rotation, eliminating wasted rotations. The best interleave is based on the speed of the particular disk drive. Interleaves are created with the low-level format.