Transform faults are places where rocks slide past each other in a lateral (horizontal) sense; these are sometimes called strike-slip faults. Transform faults do not have to be at plate boundaries. For example, the transform faults that offset parts of the midocean ridge system can occur on the same plate as well as between plates.
When transform faults are between plates they are referred to as transform plate margins. No crust is added or destroyed at these boundaries. The plates can be either two of the same kind, such as continent to continent (the classic example is the San Andreas Fault of California), or ocean to ocean such as some of the offsetting midocean ridge transform faults (that make the Frankenstein's Monster scar type pattern in the ocean ridge system) or mixed ocean and continent. As plates slide past each other they may appear to have lateral movement to either the right or the left of an observer when one looks across the fault at an object on the other side. The San Andreas Fault is a right lateral fault, and the part of California on the Pacific Plate is moving north. One day Los Angeles will be right next to San Fransisco!
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