The basic, “real” Manhattan is, as is often the case with the classics, quite simple: 2 ounces of rye, an ounce of sweet/Italian vermouth, and a couple of dashes of Angostura bitters in a mixing glass full of chunk ice; stir, strain, and garnish. The spices and sweetness of the vermouth (and don’t give in to martini thinking and skimp on it; you need the full ounce) complement the spicy, dry rye, and the bitters perk it, round it, meld it. What more would you need?
But mixologists like to fiddle with every ingredient and come up with something new — really new, not just a new name. Exchange half the vermouth for dry white vermouth, and it’s a Perfect Manhattan. Replace the bitters with Amer Picon, and it’s a Monahan. Switch the rye for Scotch, and it’s a Rob Roy.