The ultimate combination of Merino Wool for cold weather insulation and the performance that comes from a perfect fit. There is no other heavy weight sock that can make this claim. If there were we would have made it! This sock is loaded with 100% Merino Wool on the inside against your foot so all you feel is comfortable and dry. Size Chart Mens                          Ladies 3.5-5.5       Small          5-7 6-8            Medium     7.5-9.5 8.5-10       Large          10-11.5 10.5-12.5  X Large       12-14 13-15.5     XX Large     14.5-16.5 Fits Heavy Expedition Boot