The quantity of vermouth that is added to a martini determines how "dry" it is; the less vermouth in the formula, the drier the martini. You may hear someone order a martini "bone dry" and wonder if that means the drink has only gin and no vermouth at all, but this isn't the case.

A neat, chilled martini served with no vermouth is actually called a Churchill martini, named after Winston Churchill's preference for his drink. A bone dry martini simply calls for a very high ratio of gin to vermouth; anything over a 6:1 gin-to-vermouth ratio places the martini into the category of "bone dry."

A bone dry martini can also be called a Montgomery martini, after the notorious British Field Marshal Bernard Montgomery, who was rumored to enjoy his martinis at a 15:1 ratio of gin to vermouth. Today, many bartenders use a dropper or diffuser to measure out the vermouth.