A bartender at the Waldorf Astoria invented the Rob Roy in the 1890s by swapping bourbon for Scotch in a Manhattan—creating it specifically to celebrate a Scottish outlaw operetta premiering down the street. The drink survived Prohibition, two World Wars, and the vodka-soaked eighties because that one spirit swap completely transforms the personality: Scotch brings malt, mystery, and sometimes smoke where bourbon gives caramel warmth. Most people screw it up by using oxidized vermouth that's been dying on their shelf for months—vermouth is fortified wine, it goes bad, and if it's not refrigerator-fresh your Rob Roy will taste like regret no matter how good your Scotch is.