Abstract: There is provided a core lamination for electromagnetic devices. The lamination comprises an open, substantially C-shaped frame having inwardly pointing end portions facing one another across a gap, and a central limb of a substantially rectangular shape, tightly insertable with its width into the gap between the end portions. When thus inserted, the central limb defines the space enclosed by the frame two window-like openings, each of which has a height-to-width ratio of about 3:1. The shapes and relative dimensions of the open frame and the central limb are such that they are capable of interlocking and of fully tesselating a surface delimited by two parallel edges.