Tie Block
Classical guitar bridge
Slippery nylon classical guitar strings are tied onto the bridge around the upper portion which is specifically designed to grip the knot:
This "tie block" is frequently decorated with bone, ivory or plastic edges.
Sometimes the tie block has a cap of ebony or other hard dense material:
In either case, the upper corners of the block are made of very hard material and left quite square so they will resist wear from many years of string changing.