A cocktail is base spirit plus modifier plus seasoning. Bitters are the seasoning. Modifiers are the small bottles (Chartreuse, Campari, maraschino) that give liquor an identity.
What bitters do
Bitters are botanicals (bark, root, peel, spice) infused into high-proof spirit and used by the dash, like salt. They tie a drink together so it stops tasting like its parts side by side. They stretch the finish. And one dash of Angostura pulls a cloying sour back into balance.
Skip them in an Old Fashioned and you've made diluted whiskey.
The starter three
Buy these and stop. They cover most classics.
Angostura aromatic. Yellow cap, oversized label (the story goes it was a sizing mix-up they never fixed). Clove, allspice, cinnamon, bitter root. It goes in the Old Fashioned, Manhattan, and most whiskey or rum drinks. A recipe that just says "bitters" means Angostura.
Peychaud's. The bright red New Orleans bitter: lighter, more anise and cherry, less clove. Non-negotiable in a Sazerac; sub Angostura and it's a different drink.
Orange bitters. Citrus oil, dried peel, gentian. They lighten stirred whiskey drinks where Angostura feels heavy. And yes, they belong in a Martini: plenty of pre-Prohibition specs call for them. Regan's No. 6 or Angostura Orange; either works.
Later: mole bitters (Bittermens Xocolatl Mole) for stirred tequila and aged rum, celery for a Bloody Mary. Once Angostura has nothing left to teach you, Bittercube Bolivar or The Bitter Truth's aromatics spin the same idea with different spice. Skip the single-flavor long tail until a recipe asks.
How many dashes
Two dashes is the default for a stirred drink (a Manhattan). A big, heavily diluted drink gets three or four, since ice mutes bitters; a soft Martini or a Bamboo gets one. A Pisco Sour gets three to five drops on the foam, for aroma.
Modifiers
A modifier is a liqueur, fortified wine, or amaro, usually poured by the barspoon, up to ¾ oz. Vermouth and Fortified Wines covers vermouth.
Maraschino is distilled from Marasca cherries, pits and all, and tastes nothing like the red cocktail cherry. Cherry Heering won't stand in. A quarter ounce can take over a drink.
Chartreuse has the best paperwork on the shelf. In 1605 the nobleman François Hannibal d'Estrées gave the Carthusian monks near Paris a manuscript for an "elixir of long life." They fixed the recipe as the Elixir Végétal in 1764, and the liqueurs grew out of it. The monks put the count at 130 plants.
Absinthe was banned in the U.S. in 1912 on bad science about thujone, the wormwood compound blamed for "absinthism." It came back in 2007, with thujone capped at 10 ppm. Today it's a rinse: swirl a few drops to coat the glass, dump the rest. Buy Pernod Absinthe ($60) or St. George Absinthe Verte ($75), or use Herbsaint, a lower-proof New Orleans anise liqueur with no grand wormwood, made as absinthe's stand-in after the ban.
The exception to the rinse rule is Death in the Afternoon, Hemingway's contribution to the 1935 book So Red the Nose: a jigger of absinthe in a Champagne glass, topped with iced Champagne "until it attains the proper opalescent milkiness." He suggested drinking three to five. Don't.
Campari's red came from carmine, a dye made from crushed cochineal insects, until 2006 in most markets. If you like Campari's bitterness, the amari (Averna, Cynar, Fernet, Montenegro) are your next rabbit hole.
Aperol is the spritz bottle: 3 parts prosecco, 2 Aperol, 1 soda, per the IBA. In a Negroni it's a pale, sweet thing with no edge.
Orange liqueurs
Cointreau (40%): clear, dry orange peel. The standard for a Margarita or Sidecar.
Pierre Ferrand Dry Curaçao: barrel-aged and deeper. Use it when an older spec says "curaçao," as in a Mai Tai.
Grand Marnier: Cognac-based and sweeter. Fine in a Sidecar, heavy in a Margarita.
Generic triple sec: usually 15 to 25% ABV and thin. Under $10, it makes Margaritas taste like candy.
The next $300
In this order: Cointreau, Campari, Luxardo, Green Chartreuse if you can find it, a small absinthe, then Bénédictine. That builds 30+ classics. Add a base spirit (Whiskey Decoded, Gin Decoded), a vermouth, and the starter three. The rest is citrus and ice. Shake or Stir? says which technique each drink wants.