Gin, fresh lemon, simple syrup, and club soda — the cocktail that gave its name to the Collins glass.
The Tom Collins has an unusual history tied to the 'Great Tom Collins Hoax' of 1874 — a practical joke that swept American cities where people would tell a friend that someone named 'Tom Collins' had been saying terrible things about them at a bar. When the friend rushed to the bar, others would insist Tom Collins had 'just left', leading them on a chase. The joke became so widespread that the newspaper reported on it extensively.
The drink itself predates the hoax — John Collins, a waiter at London's Limmer's Hotel, is credited with the original around 1820 (made with Old Tom gin, a sweeter gin style). The 'Tom' name came from the Old Tom gin. The Collins glass — tall and narrow — was invented specifically for this drink and went on to define an entire category of long, effervescent cocktails.
Replace gin with bourbon. Named after the original bartender. Sweeter and rounder than the Tom Collins.
Replace gin with a floral gin (Hendrick's) and add ½ oz rose syrup. Serve with a dried rose petal. Pink and botanical.
Replace simple syrup with passion fruit syrup and add a splash of pineapple juice. Takes the Collins in a tropical direction.