Most whisky comes off the still at about 68% alcohol, give or take. It has been the industry standard for some years to reduce it by dilution to 63.5% before putting it in the cask. Over years of maturation, that will drop for Scotch whisky, as more alcohol evaporates than water. (Apparently, hotter conditions cause the opposite to happen in some bourbons, so that the %alc actually goes up.) When it comes time to bottle, in ten or fifteen ot twenty years, the strength in the cask may be anywhere from 50% to 60%, generally. Some casks filled at 68% or so some years ago may be higher than that; some very old casks may drop below the 40% legal threshold. Most OB's are then reduced with water to 40%, 43%, or 46%. When the bottler chooses not to reduce the whisky from the cask, it is said to be bottled at cask strength, whatever that strength may be in that circumstance. Cask strength OB's like a'bunadh or Macallan Cask Strength are usually vattings of many casks, whereas IB's are more often single cask bottlings. (G&M Cask bottlings are often vattings of two to four sister casks.) Of late, I am more fond of single cask, unchillfiltered bottlings, which often have a wonderful oily texture not found in most OB's. Bottlings from Cadenheads and the Scotch Malt Whisky Society are good examples; there are many others.