These are certification standards ratings developed for the U.S. that provide an easy reference to the strength, security, and durability of a lock. In other words, a Grade 3 lock is not as strong as a lock rated at Grade 2. As an example, for a lock to pass Grade 3 standards it must endure 200,000 cycles of opening and closing where as a Grade 2 lock must pass 400,000 cycles. The strength is also tested in increased severity as applied to forced entry through a door.