Perfect binding
Also called PUR binding, glued binding
A binding in which the gathered pages are milled at the spine and glued into a wraparound cover, giving a square, flat spine. Most paperbacks are bound this way.
In practice
It is the cheapest binding, but the book does not lie flat and, with standard hot-melt glue, can shed pages if it is forced open often. PUR adhesive holds far better, especially on coated paper, and copes with a book opened wide. Ask which glue is planned: “perfect bound” on a quote does not say.