These Field Guardian 3-in-1 Multi-Purpose Electric Fence Insulators are a multi-purpose special 3-in-1 insulator that works on T posts, wood posts, and some round posts. The multi-purpose insulators are great for high tensile steel wire, insulated electric wire, and will accept polywires and polyropes up to 3/8 in. (9.5 mm) in diameter. They can also be attached to a steel pipe/post or to any surface that will not accept nails or screws by tying with wire through a mounting hole on the back. The 3-in-1 electric fence insulators are made from a high density polyethylene material for durability.