Page "Crystallographic defect" Paragraph 16

Edge dislocations are caused by the termination of a plane of atoms in the middle of a crystal.

In such a case, the adjacent planes are not straight, but instead bend around the edge of the terminating plane so that the crystal structure is perfectly ordered on either side.

The analogy with a stack of paper is apt: if a half a piece of paper is inserted in a stack of paper, the defect in the stack is only noticeable at the edge of the half sheet.