All martinis are variations on the classic recipe. The earliest martinis emerged after the era of Prohibition, and that the first martinis contained equal parts of gin and vermouth, notes Nannette Stone, author of "The Little Black Book of Martinis." Eventually, the proportion of gin to vermouth was adjusted, and today martinis are typically made with 4 parts gin to 1 part of vermouth.

The term dry martini originally described a martini made with white vermouth, which tends to be drier than other types of vermouth. Today, ordering a dry martini is another way of asking for less vermouth. Wet martinis have a higher proportion of vermouth to gin. A standard dry martini contains 5 or 6 parts gin to 1 part vermouth.

Dirty martinis get an extra kick from the addition of olive brine. Adding olive juice brings acidity and salt to a martini. A martini can be ordered as both dry and dirty, which means you'll get a martini with less vermouth and olive juice. Dirty martinis are also typically served with one or two whole olives in the glass.

Martinis can be ordered extra dry or extra dirty. If you want an extra or very dry martini, you might make a bone dry martini, which contains 15 parts gin to 1 part vermouth. The driest martinis are made by just washing the inside of the martini glass with a light splash of vermouth before adding gin. Extra dirty martinis contain more than just a splash of olive juice -- some extra dirty martinis are made with equal parts gin and olive juice. You can also order a dirty martini with just extra olives rather than extra brine.