BS3621 is a British Standard for locks that most insurance companies have adopted in their terms and conditions. A BS3621 5 lever lock will carry the 'Kitemark' and is intended to be fitted to the final exit door of your home, assuming that this is wooden. Different standards apply to doors of different construction. A BS3621 lock has security features which make it difficult for those trying to gain access to your home illegally. 5 lever mortice locks without the BS standard are the ideal solution for medium security applications.