A zip fastener (20, 220, 320), comprising: a pair of stringers (12, 13) defining a longitudinal axis; a first slider (10, 11) disposed to slide lengthwise along the stringers and thereby join the stringers and close an opening between the stringers, the first slider (10, 11) including, at one axial end thereof, one of a male coupling part and a complementary female coupling part; a member (10, 11, 58) secured to at least one of the stringers (12, 13), the member (10, 11, 58) including, at one axial end thereof, the other of the male coupling part and complementary female coupling part; the male coupling part comprising a neck adjacent an enlarged head, and the female coupling part comprising a transverse slot with broad and narrow sections (37, 38) sized to slidably receive the head and neck (50, 51), and a shoulder between the broad and narrow sections (37, 38) of the female coupling part abuts the head to prevent axial separation of the first slider (10, 11) and the member (10, 11, 58) when the head is in the slot, and the member (10, 11, 58) is either a) a second slider (10, 11) that is disposed to slide lengthwise along the stringers (12, 13) and thereby join the stringers (12, 13) and close the opening between the stringers (12, 13), or b) a fixture on at least one of the stringers (12, 13); characterized in that the transverse slot is sized such that relative rotation between the first slider (10, 11) and the member (10, 11, 58), and resulting bending of the stringers (12, 13), allows the head to be inserted transversely into the slot, and the bending stiffness of the stringers (12, 13) tends to maintain the head in the slot. a pair of stringers (12, 13) defining a longitudinal axis; a first slider (10, 11) disposed to slide lengthwise along the stringers and thereby join the stringers and close an opening between the stringers, the first slider (10, 11) including, at one axial end thereof, one of a male coupling part and a complementary female coupling part; a member (10, 11, 58) secured to at least one of the stringers (12, 13), the member (10, 11, 58) including, at one axial end thereof, the other of the male coupling part and complementary female coupling part; the male coupling part comprising a neck adjacent an enlarged head, and the female coupling part comprising a transverse slot with broad and narrow sections (37, 38) sized to slidably receive the head and neck (50, 51), and a shoulder between the broad and narrow sections (37, 38) of the female coupling part abuts the head to prevent axial separation of the first slider (10, 11) and the member (10, 11, 58) when the head is in the slot, and the member (10, 11, 58) is either a) a second slider (10, 11) that is disposed to slide lengthwise along the stringers (12, 13) and thereby join the stringers (12, 13) and close the opening between the stringers (12, 13), or b) a fixture on at least one of the stringers (12, 13); characterized in that the transverse slot is sized such that relative rotation between the first slider (10, 11) and the member (10, 11, 58), and resulting bending of the stringers (12, 13), allows the head to be inserted transversely into the slot, and the bending stiffness of the stringers (12, 13) tends to maintain the head in the slot.