These stainless steel thimbles are made to exacting tolerances. The purpose of a thimble is to form a small neat loop (the loop is crimped to keep securely positioned) the thimble will then protect the mono on the inside of the loop from damage if it is put in contact with a swivel, snap-link etc. Use small for mono in breaking strains above 200lb and medium for mono above 300lb. These thimbles come in a packet of 10.