Weighing only 36g per metre, the Beal Gully 7.3mm is the lightest half rope on the market (as of Christmas 2016).
Its extremely light weight and very low impact force make the Gully 7.3mm the ideal rope for ice climbing, mountaineering and the most demanding high altitude terrain. Like all other thin half ropes in Beal's range, the Gully 7.3 benefits from the reliability and additional safety that Unicore technology provides. Unicore is a process that bonds the sheath to the core in such away that if the sheath is cut or damaged there'll be no bunching up when abseiling, ascending or belaying. This means that you get zero sheath slippage. It is also certified as twin rope.
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Warning: Extreme care is required for both belaying and abseiling. The very thin diameter requires the use of compatible devices and limits its suitability to experts who are familiar with thin, slick ropes.
NB This a 'Half Rope' and should be used in conjunction with another Half Rope. Confusingly these are also sometimes known as 'Double Ropes'.
Having said that, people often use Half Ropes singly, particularly in 25m and 30m lengths, for scrambling (to protect the second, not the leader) and for glacier travel where a fall is likely to be more of a slither. These are situations for which there are no official tests and individual judgement must be used.
Price is per rope, not per pair (except for the pair deal, obviously!).