Most transform plate boundaries occur in the oceanic lithosphere where they connect segments of ridges (spreading centers). The image above is of part of the Pacific Ocean floor, lookong toward Central America.  The ridge shown is called the East Pacific Rise.  The yellow arrows indicate spreading directions.  Many tranforms can be seen that connect short segments of the ridge.  Since the two lithospheric plates slide past one another along the transforms, these boundaries are active seismic zones, producing numerous shallow eartquakes. 
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Some transform plate boundaries pass through continental crust.  An example of such a transform is the San Andreas Fault. Along the San Andreas Fault the Pacific plate moves in a  northwest direction relative to the North American plate. The transform connects two oceanic ridges (spreading centers) the East Pacific Rise (seen in the picture at the top of this page) and the Juan de Fuca.  You can see this, if you enlarge the picture below-right.
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Divergent Plate Boundaries
Convergent Plate Boundaries
Transform Plate Boundaries Some features of ocean basins