Trimming the edges of a book using a plough is an ancient yet effective technique, but getting the book positioned correctly in the laying press so the cut is accurate can be a frustrating part of the process. A ploughing aid can help this by setting up the position of the book out of the press and guaranteeing an accurate cut. As far as I know no-one produces these commercially (yet), but they are relatively simple to make and an absolute boon if you use a plough! (Credit to my friend Bob Hamer who made this one for me.) Footnote: I understand from others that the original design for this simple but effective device was probably created by a John Alabaster here in the UK in the late 1990's. We are indebted to his ingenuity!