A joint comprising: a structure; a fastener passing through the structure with an end of the fastener protruding from the structure, the end of the fastener comprising: an axially extending shaft, a fastener head carried by the shaft, and one or more washers located between the fastener head and the structure, the one or more washers spacing the fastener head and the structure apart by a distance; a cap having a cap body with a cavity which encloses the end of the fastener; and a cured sealing material which seals the cavity, wherein: one of the fastener head and the cap body is formed with two or more first locking features (143, 123) which are axially spaced apart by a pitch, and the other of the cap body and the fastener head is formed with one or more second locking features (123, 143) which are each capable of interlocking with one of the first locking features; and at least one of the second locking features is interlocked with a respective one of the first locking features to secure the cap to the fastener head, characterised in that the distance between the fastener head and the structure is an integer multiple of the pitch between the first locking features. a structure; a fastener passing through the structure with an end of the fastener protruding from the structure, the end of the fastener comprising: an axially extending shaft, a fastener head carried by the shaft, and one or more washers located between the fastener head and the structure, the one or more washers spacing the fastener head and the structure apart by a distance; an axially extending shaft, a fastener head carried by the shaft, and one or more washers located between the fastener head and the structure, the one or more washers spacing the fastener head and the structure apart by a distance; a cap having a cap body with a cavity which encloses the end of the fastener; and a cured sealing material which seals the cavity, wherein: one of the fastener head and the cap body is formed with two or more first locking features (143, 123) which are axially spaced apart by a pitch, and the other of the cap body and the fastener head is formed with one or more second locking features (123, 143) which are each capable of interlocking with one of the first locking features; and at least one of the second locking features is interlocked with a respective one of the first locking features to secure the cap to the fastener head, characterised in that the distance between the fastener head and the structure is an integer multiple of the pitch between the first locking features. one of the fastener head and the cap body is formed with two or more first locking features (143, 123) which are axially spaced apart by a pitch, and the other of the cap body and the fastener head is formed with one or more second locking features (123, 143) which are each capable of interlocking with one of the first locking features; and at least one of the second locking features is interlocked with a respective one of the first locking features to secure the cap to the fastener head, characterised in that the distance between the fastener head and the structure is an integer multiple of the pitch between the first locking features.