I have a pass-fail test for bartenders: I sit at the bar and order a Manhattan. Does the barkeep ask whether I would prefer bourbon or rye? Has the vermouth been properly chilled? Has the drink been stirred instead of shaken?

But perhaps the most important question of all is this: Were two dashes of bitters added to the mix before stirring? If not: fail.

It seems straightforward. Just about all of the classic straight-up cocktails call for a dash or two of some sort of bitters, be it Angostura, orange or Peychaud’s. Why, then, are bitters so often ignored or forgotten?

Not long ago, I was at a popular sports-ish, Irish pub-ish bar in Dupont Circle that happened to have a Manhattan on its drinks list. Against my better judgment, I ordered one, specifically asking for a couple dashes of bitters. “Uh, bitters?” the bartender replied. “I don’t think we have bitters here.” He went off to ask his manager and came back. “Yeah, we don’t carry bitters here.”

Bitters are among the ingredients most baffling to cocktail newbies, and I empathize. They’re just weird. They come in tiny bottles with dasher caps that prevent a normal pour. Consider the most prevalent, Angostura bitters, the one with the label that appears to be too big for the bottle. It’s one of the few bar staples that you can buy in a supermarket, even though it’s pretty potent, at just under 90 proof.

Toronto cocktail photographed in Washington, DC. (Deb Lindsey/FOR THE WASHINGTON POST)

When I make cocktails for a group of people, someone inevitably stops me and asks, “Wait, what did you just shake into that drink?” For that reason, I was happy to read Brad Thomas Parsons’s facinating new book, “Bitters: A Spirited History of a Classic Cure-All” (Ten Speed Press), which attempts to demystify these mysterious elixirs.

“One of the biggest misconceptions about bitters is that using them will make your drink bitter,” Parsons writes. Instead, he calls bitters a “liquid seasoning agent” that acts as a sort of “bartender’s salt and pepper.”

“Bitters are the ultimate matchmaker: Just a dash or two can bring a perfect balance to two seemingly incompatible spirits,” Parsons writes. A dash of bitters, for instance, will often bring complexity and balance to a sweet or fruity drink.

When we talk about bitters, we must be clear. The bitters we’re talking about, the ones in the little bottles such as Angostura or Peychaud’s, are correctly referred to as aromatic bitters. Parsons is careful to draw a distinction between these and what he calls “potable” bitters. Those would be the category of drinking spirits that Italians call amari: Campari, Averna, Ramazzotti, Fernet Branca.

There are some similarities between the two types of bitters. Both are made from an infusion of roots, barks, fruit peels, spices, herbs and flowers, and both were created for medicinal purposes. In 19th-century America, bitters were a booming business, with snake-oil salesmen promising cures for colds and coughs, fever sores, jaundice and flatuence. Step right up, sir, your morning cocktail is now a health drink!

The proliferation of bitters died off during Prohibition, and over the course of the 20th century, the correct use of aromatic bitters — like everything else about good cocktailmaking — began to wane.

That is no longer the case, as cocktail geeks like me have demanded their bitters. Over the past five years, a slew of new bitters, from companies such as Fee Brothers, the Bitter Truth and Bittermens — not to mention numerous bartenders creating their own house-made bitters — have ushered in a veritable Bitters Renaissance. Now, you can find peach, grapefruit and rhubarb bitters. Certain varieties, such as Fee Brothers Whiskey Barrel-Aged bitters and the Bitter Truth’s celery bitters and Bittermens Xocolatl Mole bitters, have quickly become new classics.

Parsons chronicles all of that, offering techniques for making bitters at home as well as a great collection of unique cocktail recipes.

He even deals with the Great Angostura Shortage of 2009-2010 and offers two pieces of lore explaining why the Angostura bottle bears an oversize label: In one, the wrong label was applied and never corrected; in the other, the person ordering the label never communicated with the person ordering the bottles.

Two of my favorite recipes from Parsons’s book accompany this column. In the Toronto , a Manhattan variation that originally called for Canadian whisky, you will find aromatic bitters and potable bitters (in this case, Fernet Branca) at work. In the Sawyer, the aromatic bitters take center stage in a gimlet variation that calls for a whopping 28 dashes of Angostura, Peychaud’s and orange bitters combined.

That is a certainly a drink for which ignoring the bitters will result in immediate failure.

Wilson is the author of “Boozehound: On the Trail of the Rare, the Obscure, and the Overrated in Spirits” (Ten Speed Press, 2011). Follow him on twitter @boozecolumnist.