Measuring the joint width made simple. Schmedt. The World of Bookbinding.
The spine gauge for bookbinders is a simple tool to measure the width of the spine hollow for a book case. Scissor-like tool. The side with the narrow opening is closed around the book block. The other side shows the correct spine hollow width for this book block by its opening width. More on: https://www.schmedt24.de/en