Apart from the dictates of etiquette, does it really make a difference whether you serve tea in coffee cups or sip your coffee from a teacup? The answer is yes. Basically, a tea cup is wider at the mouth, often more tapered, and usually shallower, while a coffee cup is defined by a more upright form.
Cup styles are attuned to the specific nature of the beverages they serve, so the properties of tea and coffee dictate the cup shape.
Tea should be allowed to cool slightly as it’s enjoyed, and is sometimes brewed in the cup with an individual tea bag – and brewing water must not be boiling when the tea is added, as it can subtly ruin the flavor. Wider, shallower tea cups allow the temperature to change at the required rate for a perfect cup.
Coffee is generally preferred very hot (and higher temperatures peak its flavor instead of damaging it), so the definition of a coffee cup – higher-sided with a narrower mouth than a teacup – pairs the beverage with a vessel that best conserves its heat.
Tea needs to mingle with air to fully develop its aroma and flavor, so tea cups have more open mouths so that tea molecules will contact the air and the delicate nuances can be more easily appreciated by the drinker.
Coffee‘s bitter, dark flavor doesn’t require such open vessels; the narrower mouth of a coffee cup creates a column of aroma that lets you appreciate the strong flavor on your tongue instead of as a scent.
These rules hold true whether you’re using full-sized or demitasse cups.
Would you prefer a single cup that lets you serve either drink without mismatching your tea table? While it’s not quite as traditional, a slightly larger breakfast cup with a distinct “pinch” or “waist” in its body balances the properties of the teacup and the coffee cup for an elegant-looking compromise.