One ounce, occasionally less, is the standard fill line for the small pleated paper cup handed over at the end of a cleaning, a volume set less by any dental standard than by the size of cup that stacks efficiently in a wall-mounted dispenser designed originally for water coolers. The mouthwash inside is rarely the same brand twice between visits and rarely chosen by the patient: whatever concentrate the office buys in bulk, diluted to whatever ratio the assistant on duty was taught, poured to the cup's printed line or eyeballed close to it. The cup itself pleats along the sides in a fan pattern that lets it collapse flat for storage and pop back into a cylinder under light pressure, a piece of paper engineering solved early in the last century and never meaningfully improved since.
It holds liquid for exactly as long as it needs to, a minute at most, the interval between being handed over and being spat into the small chrome bowl fixed to the chair, after which it is crushed once, out of habit more than necessity, and dropped in the same motion as the bib and the straw sleeve. Some offices switch to a stronger, medicinal rinse for patients who have just had a filling. Few patients can say afterward what flavor it was, only that it stung faintly at the back of the throat, or did not. Nobody asks for a second cup. The dispenser holds four hundred more, and will be refilled on Thursday whether or not it has been emptied.