When distilling Scotch on a pot still, the distillate (“new make”) can be anywhere from 64% to 75% ABV; on a column still, the proof could be even higher.
This distillate could enter the barrel for aging at any of these proofs. Yet in practice, most Scottish distilleries add water until the distillate is at 63.4% ABV, before barreling.
This is a common practice, yet not a law. So why dilute the distillate at all? Wouldn’t it be cheaper to enter the higher proof distillate directly into a barrel? That way one would need less barrels (and this would save a lot of money.) And even if there is a reason to dilute the distillate, why are most Scotch distillers diluting it down to the same ABV.
Distilleries do this mostly because of
a) it’s an abv that allows the spirit to evolve very evenly during maturation,
and b) most distilleries are producing whisky to trade with other distilleries to fulfill needs for their blends which make up around 90-95% of all scotch whisky sales.
Because the angel’s share in Scotland is very even (around 1.7% volume year over year), GlenWhisky can confidently trade their 12 yo with GlenHooch for their 12 and count on basically getting the same volume and same ABVs.
Some distilleries fill at a higher ABV such as Tamdhu and occasionally Glenfarclas, Loch Lomond, and Ben Nevis. That ABV is between 70-71% for these distilleries.
Tamdhu is ALWAYS 70-71% while the others fill only occasionally at this higher ABV. This is typically not done as higher ABVS tends to produce lighter, fruitier esters which simply may not be in the distillery wheelhouse/desired flavor profile.
Distilleries do not fill lower because they lose ABV over the years. Barreling at 63.4% allows Scotch whisky to develop in a used cask for many many years, allowing for the ABV to remain above 40% for decades.