A contact fuze, percussion fuze or direct-action (D.A.) fuze (UK) is the fuze that is placed in the nose of a bomb or shell so that it will detonate on contact with a hard surface. Many impacts are unpredictable: they may involve a soft surface, or an off-axis grazing impact. The pure contact fuze is often unreliable in such cases and so a more sensitive graze fuze or inertia fuze is used instead. The two types are often combined in the same mechanism.