A salty dog is a cocktail of gin, or vodka, and grapefruit juice, served in a highball glass with a salted rim. The salt is the only difference between a salty dog and a greyhound. The Greyhound dates to at least 1930, when it appeared in Harry Craddock’s "Savoy Cocktail Book" as a simple cocktail combining gin, grapefruit and ice.
Fill the glass with ice, then add the gin (or vodka) and grapefruit juice and stir gently to combine.
Garnish with a grapefruit slice.