The strap I purchased was one of two sizes. The website gives the 'sizing' information below. The sizing determined I required the extra large (above 13.5" / 34cm) which my wife and I confirmed three times as the sizing included the warning 'as once opened these items cannot be returned'. It turns out the extra large is much too large so, having followed the sizing process, I have wasted £29. Not a happy customer Geoff Glinternetorders@yahoo.co.uk Sizing up for your PureSom Ruby Chin Strap Adjustable To get the correct size of Ruby Adjustable Chin Strap, use a measuring tape from middle of the chin, past the ear, to the top middle of the head. Those with sizes up to 13" / 33cm should choose the Regular size, while those above 13.5" / 34cm should choose Extra Large. Please ensure that you use this guidance, as once opened these items cannot be returned for hygiene reasons.