Building An Old-Fashioned
Here is a typical way I make a fine cocktail.
I start with a complex personal blend of bourbons, straight ryes and Canadian whisky. Usually this has some sweet element, it may come from maple syrup, Grenadine, or Rock 'n Rye or (usually) a combination.
I pour this on ice. Swish around. Taste. It seems a little lean on bourbon character tonight. I add good bourbon, some 1980's-era Old Taylor and 12 year old Old Charter, tonight.
Hmmm, the bourbon character is good now but it needs some citrus, for the Old-Fashioned effect. I add two slices of Mandarine orange. Now we're getting there. But, maybe the taste is a bit bland: I add two drops of Collins orange bitters. Now we've got it, the Collins is contrapuntal to the sweet in the glass, it's starting to rock.
I'm almost there but ... somehow it isn't sweet enough, or the balance ain't right. A good iced Old-Fashioned needs a constant undertone of sweetness.
I add a dash of Grenadine. Now I've got it! The perfect pre-prandial drink. Lightly sweet, but with complex straight whiskey character - satiating, but I don't need another.
The art of the cocktail: exemplified.
Presentation vs Comination
In response to Scratchline
I think all cocktails are designed to accomplish two things:
The more I look into it, the more I feel that the Old-Fashioned is not so much a "cocktail" as a mode of presentation. Notice that some recipes call for liquor of choice prepared in the old-fashioned style. Could be that it was an early attempt to make rough liquor more palatable with the addition of a few different flavorings.
1. Increase Palatability: visual, taste, texture and smell
2 Increase rate of absorption: Diluted spirits in acidic solutions are absorbed most rapidly
The more obnoxious the base spirit, the more varied the additions to overcome the character. Bad whiskey is surely a primary reason for the cocktail.