A closure is provided and comprises a top plate and a sidewall depending therefrom. The interior of the sidewall has a screw thread formation comprising a plurality of thread segments. The thread segments collectively define an engagement surface for engagement with an external thread formation on an associated container. A notional helical top surface with a constant pitch extends along the formation, and at least some, but not all, of the segments include material offset from the notional helical top surface towards the top plate.