The back square is the part of the end of the blade where the spring sits when the knife is open:
- When modelling in plastic for accuracy it is best to aim for an exact fit between the back square and spring, any slack here will result in up and down blade wobble when open.
- When working in steel you might wish to gently round the corner of the back square. This allows some space between the two so the knife will still open accurately if dirt accumulates here.
- Be aware that too much shaping here will lead the spring to be sat on just two points of the blade and just subtle changes to these will radically alter how the blade sits in the handle.
- In the closed position there is a tendency for the back square to protrude from the handle - this makes a sharp corner which will not be pleasant in the pocket. If possible the handle should be shaped to cover it although in some designs it is unavoidable:
