Shaken or stirred?

 A true martini is a very different drink from the other drinks one gets in a bar. 

Walk into a bar and order a whiskey. In England before they went metric (one of Orwell's great fears), the bartender would pour exactly 1/6 gill of the house Scotch into a glass. One could drink the Scotch neat, or order a mixer and add it to the glass (that would hold the 1/6 gill and an English bottle of mixer (English bottles of mixer are rather small). Most alcohol authorities specify exactly how much of a spirit to serve, and tax that quantity. Since the metric system hit the British pub, an order of whiskey is either 25 ml or 35 ml (depending on the pub). In the US, which have more than 50 different alcohol authorities, an order of whiskey might be 1 ounce, 1.25 ounce, or 1.5 ounce by law. Maybe with the option of a small, medium, or large whiskey, or maybe not, depending on the alcohol authority. So, order a whiskey in the US and get exactly 1, 1.25, or 1.5 ounce of the house Bourbon poured into a large glass into which one normally adds ice and a mixer.

Order a cocktail, and the bartender mixes the appropriate spirit and liqueurs in the prescribed quantities and pours the chilled mixture into a glass that is larger than the amount of the cocktail (so the bartender has room for an umbrella or other froufrou).

But a martini is a mixture of gin and French-style vermouth that must be poured into a martini glass up to the brim. If the bartender has more martini left over, it is thrown away or sold to someone else. One martini glass filled to the brim is what one gets when one orders a martini. Martini glasses vary in sizes, and how much of what is in the glass varies from bar to bar.

Vermouth was invented by the Italians, who created a very sweet, red liqueur. They were followed by the French who created a yellow, dry liqueur. Today, the Italian Martini company makes about 5 different kinds of vermouth, some Italian style, some French style (I haven't found any French companies that make Italian style vermouth).

Because the martini glass must be filled to the brim, one wants it filled with the best ingredients. In the US, the tax on gin is about 10 times the tax on vermouth, so the gin is the most expensive ingredient, and vermouth is the 2nd. The gin and vermouth must be put into a cocktail mixer with ice. The more French vermouth, the more yellow the martini, and they buyer could see it had a lot of the cheaper vermouth. So Martini came out with a clear, dry vermouth, so French style, but clear, so a martini with 50% clear Martini vermouth (or 75% Martini vermouth) looks like it is almost all gin, which is why many bars switched to the clear Martini dry vermouth so they could make a cheap martini that looked expensive.

But, of course, the cheapest ingredient is water. The bar could use bar freezer ice, 0°F (-17.8°C) which barely melts, or ordinary freezer ice, 30°F (-1°C) that melts. A lot. And makes the drink watery. And one can stir, which minimises the melting of the ice, or shake, which maximises the melting.

It is said that one martini drinker demanded they pour a bit of French vermouth into the martini glass, then pour it all out, then fill the glass with gin chilled with bar freezer ice for a very short time, and barely stirred, never shaken. So his martini glass had just about all gin (and the requisite olive).

Most martini drinkers want a martini that is almost clear (so almost all gin) and stirred, not shaken, so they get as much gin and as little water as possible in their martini glasses.

In the first Bond novel, he calls for a drink he calls a martini. It consists of gin, vodka, and a kind of French vermouth that was discontinued in 1986 (that vermouth contained quinine, useful if one had malaria before they came out with more modern drugs to treat the disease). The mixture was well shaken with lots of ice, then poured into a large goblet. So not a real martini at all. Sounds like it was once a very nice drink, but it was never a real martini.