We don’t own any of these dainty cups & saucers. But we’d politely serve guests with the saucer if we had any. 
Why are mugs exempted from the saucer etiquette rule? Is it a faux pas to serve guests coffee in a mug?
Somebody probably would consider it a breach of etiquette. I would imagine, if it’s fancy enough to warrant a cup and saucer, you don’t want drips puddling on the table and the saucer also functions as a coaster.
My family has always been a mug family.
tea cups are top heavy and a saucer adds stability. also the thinner end allows dribbles with the dainty drinking methods.
mugs are stable containers with a thick non-drip edge.
Someplace to put the spoon after they doctor it to their tastes
Mugs are for husky German he-men to drink BEER out of in the comaraderie of their fellow husky German he-men in the Rathskellers.
Drinking coffee out of a mug was just an afterthought.
Saucers are also used to serve the tea, making it much easier to place on the table without spilling.
I collect a few pieces of vintage restaurant ware. I bought several Victor insulator restaurant coffee mugs from Ebay a few years ago. Mine date back to the 30’s. We use them daily. Reminds me of the mom & pop cafes where grandad had his morning coffee. I’d go with him sometimes. I love thoseold mushroom shaped mugs. A piece of my childhood. They keep the coffee hot too.
They are too big to fit on a saucer.
Yeah, what is it about those mugs? Coffee and tea both seem to taste better in them. Probably because I know I’m only gonna get 6-ish ounces. 
Coffee cups are for serving, and can allow the person you are serving to mix in their own additions without spilling and provides a place for spoon, napkin and maybe even a cookie.
Mugs are self-service devices for casual personal use. You are supposed to do your milk and sugar in the kitchen where you poured it.
practical reason: someplace to put your spoon without leaving a mess.
esoteric reason: tea/coffee expensive and only for the wealthy in the 17th/18th century. Lowly plebians drank cheap grainy beer out of clay mugs, rich people drank out of expensive porcelain cups placed onto expensive porcelain saucers. The nouveau riche merchant class wanted to mimic the upper class so they adopted the cup/saucer as well.
That’s an interesting point. I’ve seen coffee served at the table and people add their cream/sugar. Then they have a wet spoon to deal with. I fill my coffee mug on the kitchen counter.
Post 4!
Thanks AncientHumanoid. I drink my coffee black and didn’t think about the problem of a wet spoon.
I like my coffee like I like my women…
I like my women like I like my coffee.
hot and wet?
Covered in BEES!
Strong, black, and sweet
Hot and creamy
Cold, greasy, and with odd lumps
Tied up in a sack & thrown over the back of a mule?
Dark and bitter?
The practical consideration of the tea cup is to make it small so that one can fit more tea cups into one box, like for a picnic set.
Of course, the etiquette then goes on to require the saucer, so that the cup can be wet on the outside, due to spills and drips, but the saucer’s bottom is kept dry. This would be important to avoid brown stains on white clothing…