(THE WASHINGTON POST) - They are the zipper on the jacket, the key to the door, the fingerprint that makes or breaks the case. They are usually the smallest component of the drink in your glass, but if you leave them out, you can end up with a spineless mess.

Cocktail bitters evolved out of archaic medicines made from plants believed to have pharmaceutical properties, once used to treat all sorts of ailments. And even if you are not much of a drinker, you have probably seen at least two of their oldest and most well-known delegates hanging out near the tonic water and neon maraschino cherries at the grocery store: Angostura bitters, sporting a weird oversize label and chipper yellow cap, perhaps alongside a slightly less ubiquitous friend, the brilliant red Peychaud's bitters. Both date to the 19th century.

These are just two among what has become, over the past 15 years or so of the cocktail renaissance, a vast labyrinth of "non-potable" bitters. This odd term means bitters are not designed to be consumed on their own, but used as flavouring - much like vanilla extract, which has a similar alcohol level but which most sane people would not want to drink straight. Their non-potable classification is why bitters may be sold in stores that do not sell hard liquor.

Although any bottle labeled as "bitters" will usually have a bitter component (typically from a botanical element such as gentian, cinchona bark, wormwood or the like), the bitterness is a base for a range of other aromas and flavours. You can find bitters that taste of flowers, of tea, of citrus and pepper and spices, of leather, of nuts, of jerk seasoning, of smoke and of combinations of these. Ginger and lemon. Coffee and cocoa. Crawfish boil.

I counted recently, and I have some 35 commercial bitters on my shelves, a tally that will make me seem like a crazy hoarder to some and a dilettante newbie to others (I know home cocktailers who have a hundred or more).

How many of these bitters do I use regularly? Maybe four. The others, here and there, once or twice a year.

"There's a lot of noise in the category now, and you're dealing with a product that will last you forever and ever and ever," says Brad Thomas Parsons, author of Bitters: A Spirited History Of A Classic Cure-All and the more recent Amaro.

"Unless you're a bar that's going through a lot, if you buy a bottle of bitters for home use, you're going to have it for a while."

They will last you forever because most cocktail recipes call for mere "dashes" of bitters, a measurement that, depending on the bottle you are dashing or dropping from, may be anywhere between a few drops to an eighth of a teaspoon.

So, given your limited space and budget, which ones should you buy? Are they even necessary? The latter question I will answer with a definitive yes: You need bitters to make some of the best-known classic cocktails; there are drinks that are not the same without them. They bind, they brace, they emphasise flavour notes in spirits dark and light. They are like Lebowski's rug: they really tie the room together.

As to which ones you need, that will depend on how far down the rabbit hole you want to go.

Stand in front of the offerings at Amor y Amargo ("Love and bitters" in Spanish), a tasting room and cocktail bar in New York's East Village, and you might feel a little overwhelmed. Beverage director Sother Teague estimates he has around 200 bitters on the bar, and guests can taste the bitters on the bar, sample them in drinks, and buy ones they want to use at home.

While few bars have that kind of retail capacity, many friendly bartenders will be all too happy to oblige a guest who politely asks to taste a bitters or two; just do not go overboard or ask to do this when a bar is three deep with customers howling for drinks.

When you are tasting bitters, Teague says, pick up the bottle and give it a quick snap to create volatility and release the aromas. Open the bottle and smell it directly from the bottle, then put a drop on the back of your hand and taste it with the wide part of your tongue, rolling it around on your palate. With their high proof and loads of spice, bitters blast the palate, so you will want to cleanse yours between tastes.

"It's important to smell them, get the aromatics, rub them between your palms and inhale," Parsons says. "Try them in soda water so you can taste them potably rather than just getting this hot alcoholic thing on your tongue."

Most cocktailers seem fairly consistent about where to start a bitters collection: with Angostura, Peychaud's and an orange bitters. Teague makes a culinary comparison: "I was a chef for 12 years, so I say all these bitters on this bar, these are my herbs and spices. But Ango is salt," he explains. "I know the math doesn't add up, but Ango, Peychaud's and orange bitters - that's your salt and pepper. A chef can do a lot of things with rosemary. He can't do a damn thing without salt."

Angostura, Peychaud's and many others are labeled as "aromatic" bitters, which can be confusing. One of the main functions of any bitters, whether labeled "aromatic" or not, is providing aroma. Generally, Parsons says, most newer "aromatic" bitters tend to echo or riff off the cinnamon-y, cardamom-y spice mélange of scent Angostura is known for, without necessarily having one preeminent flavor.

Daniyel Jones, global brand ambassador for House of Angostura, said via email that he loves Ango's ability to temper anything acidic or astringent. Take a classic daiquiri, he says, of rum, sugar and lime and add a couple of dashes of Angostura. "It tempers the lime yet highlights the bright refreshing citrus and rum. . . . I challenge you, next time you have a daiquiri, try one with and one without the bitters and see which you prefer."

Peychaud's, which is also labeled "aromatic," is a dramatically different beast - prominently anise-y and a bit minty. Its uses are somewhat more limited (fewer cocktails call for it), but if you're a fan of Sazeracs, it is a must-have. As with Angostura, other bitters hit some of the same flavour notes; for example, the Creole Bitters made by the Bitter Truth can be a fine substitute.

Regan's Orange Bitters No. 6 is often the third (and the youngest) member of the trifecta; introduced in the mid-2000s, it is orangy with a funky cardamom-y backnote. Fee Brothers, the Bitter Truth and Angostura all make an orange bitters as well, so you may want to test and see which you like best; some bartenders have been known to mix them to hit a house orange bitters that has the notes they want.

If you want to branch out further, Parsons and Teague are consistent in recommending the second step: a mole and a grapefruit: "Mole for your brown spirits; then the other side of that coin is a grapefruit bitters from Scrappy's, which is delicious. That's for your gin, your white rum, your blanco tequila, your aquavit, your cachaça," Teague says.

If you get past this level, you are probably well prepared to explore on your own, focusing on adding to the drinks you've tried and want to make at home. And next time you are making a Manhattan or martini, try adding a different bitters to it. You may discover a whole new angle on a cocktail you thought you knew.

Shaking up two classics

If you're a Manhattan drinker, make the classic drink but change up the bitters - try: - 2 dashes mole bitters, 1 dash orange bitters - 1 dash orange bitters, 2 dashes pimento bitters - 2 dashes cherry bark-vanilla bitters, 1 dash chocolate bitters

If you're a Martini drinker, try: - 1 dash orange bitters, 2 dashes grapefruit bitters - 2 dashes celery bitters, 1 dash lemon bitters - 2 dashes lavender bitters, 1 dash orange bitters.