Castle nut has slots (notches) in the end, which look like castle. Castle nut design helps to lock the bolt or stud. Just use split pin or cotter pin, R- pin for safety and pass through the hole. Castellated nuts are used in low-torque applications, such as holding a wheel bearing in place.Mild Steel (M.S) Black or Galvanized (G.I) Surface Grade 8.8, 10.9 Available from 10 mm till 52 mm, 3/8" till 2" and as per requirementStainless Steel (S.S) Grade 304 and 316 Available from 10 mm till 52 mm, 3/8" till 2" and as per requirement