Spanners come in all shapes and sizes, many being developed to deal with a specific job. By far the most important consideration when using a spanner is to ensure that it fits the nut perfectly. Too loose, and it will round the comers of the nut - and slip, often damaging the nut. Spanner and your hand. When purchasing spanners, select good quality tools. The best types are forged from carbon steel or chrome vanadium, if not abused, these will last a lifetime.