What classifies an 'original' martini?

A classic, original martini consists of dry gin and dry vermouth. Stir with ice and strain into a martini glass with an unstuffed Spanish olive. 

Is the 'dry gin' important?

Hell yes. A classic martini uses not just any gin, it uses a London dry gin. This does not mean the gin needs to be made in London, however! The term “London dry gin” means that it follows a specific process that ensures the quality of the gin.

What about the ratio?

The ratio of vermouth to gin can be no more than 1:1 (and is usually less).

Does size matter?

Of course. A classic drink is all in the detail A classic martini is purely alcohol, so a little goes a long way. A classic martini is usually a delicate 100-120mls. 

Which glass? 

It's in the name – a Martini glass! It is a perfectly conical bowl tapering down to a thin, footed stem. Although you might see oversized martini glasses, save those for variations of martini cocktails and use an appropriately sized martini glass for a classic martini.

And to finish?

Most commonly, the classic garnish is an unstuffed Spanish green olive either in the drink or on a skewer. But even purists are known to branch out with a stuff olive or even use garlic, jalapeño, or a cube of cheese.

What Is a dirty martini?

Adding a splash of olive brine is what turns a classic martini into a dirty one. The result is a cloudier liquid finish, hence the name 'dirty'.  

Another popular martini garnish, however, is “with a twist.” This is a small bit of lemon peel, which is twisted to release the oils. It provides a nice citrusy pop.