Demerara sugar is partially refined cane sugar, with a natural amber colour and a faint molasses character that bright-white refined sugar does not have. Dissolved 2 : 1 with water, it makes the syrup that lifts an Old Fashioned out of the merely-sweet — and that every serious tiki recipe assumes you have on hand.
Why 2 : 1, Not 1 : 1
The doubled sugar concentration does two things. First, the syrup is thicker — it brings body and mouthfeel to a stirred drink in a way thinner simple syrup can't. Second, a quarter-ounce of 2 : 1 syrup sweetens about the same as a half-ounce of 1 : 1, which keeps spirit-forward drinks from getting watered down by extra liquid. For an Old Fashioned, that distinction matters: you want sweetness without dilution.
Why Demerara Specifically
White sugar at 2 : 1 just makes thick simple syrup. The molasses notes in demerara — caramel, dried fig, a hint of toast — are the entire point. They flatter aged rum and bourbon, they take well to bitters, and they give a stirred drink the brown-sugar depth that ordinary sugar cannot.
Sourcing
Look for true demerara — large amber crystals, sold under that name or as "raw cane sugar." A lot of supermarket "demerara" in the U.S. is white sugar tinted with a wash of molasses; the difference is real. Turbinado is a closer cousin and works in a pinch.
Shelf and Storage
The 2 : 1 ratio is mildly self-preserving — the higher sugar concentration discourages fermentation. Refrigerated in a clean jar, expect about three weeks before the syrup starts to cloud. A small splash of vodka (a teaspoon per cup) extends that further if you make a larger batch.
Bottom Line
If you stir spirit-forward drinks at home — Old Fashioneds, anything in the Manhattan family, dark rum cocktails — this is the second jar (after simple syrup) to keep in the fridge. It earns its place quickly.