We describe what is probably the most common way of working a square corner when covering the boards of a book with bookcloth or paper. There are many others, but the popularity of this one confirms it as the best in many ways.
We show it, below, by using bookcloth, but it will work with anything, even more fragile coverings such as paper. The method does result in several thicknesses of covering material, and so thicker stuff is best treated some other way. Leather, for example, can be pared down or butted with an exact mitre. Building-up the inside of the cover board is another option. All these latter points fall into the realms of bookbinding, my emphasis throughout this site is for bookcraft. What is the difference? It is a fairly long waffle on my part..another time...another place...[xxxxxx].