The diagram shows the effect of 'sliding' an Edge (a), Screw (b), or Mixed (c) dislocation completely across a slip plane in the crystal. Each dislocation has the same effect as it moves completely ...

For the tilt boundary shown, the dislocations are straight edge type. For a pure twist boundary, an array of screw dislocations is required. From: Guy & Hren, "Elements of Physical Metallurgy," ...