of the tea set in the 18th century. At that time, the spoons were playing important role in the tea drinking etiquette: a spoon laid "across" the teacup indicated that the guest did not need any more tea, otherwise, the hostess was obligated to offer a fresh cup of tea, and it was considered impolite to refuse the offering. Pettigrew reports that sometimes the spoons were numbered to make it easier to match the cups with the guests after a refill. Unit of measure In some countries, a teaspoon (occasionally "teaspoonful") is a cooking measure of volume, especially widely used in cooking recipes and pharmaceutic medical prescriptions. In English it is abbreviated as tsp. or, less often, as t., ts., or tspn.. The abbreviation is never capitalized because a capital letter is customarily reserved for the larger tablespoon ("Tbsp.", "T.", "Tbls.", or "Tb."). The household teaspoons provide very bad approximations of any unit of measure. In a small-scale research, Falagas et al. found out that the volume of liquids inside different teaspoons varies almost three times, between 2.5 and 7.3 ml (the average value was 4.4 ml). Metric teaspoon The metric teaspoon as a unit of culinary measure is 5 ml, equal to , UK/Canadian metric tablespoon, or Australian metric tablespoon. United States customary unit As a unit of culinary measure, one teaspoon in the United States is tablespoon, exactly , 1 US fluid drams, US fl oz, US cup, US liquid gallon, or (0.30078125) cubic inches. For nutritional labeling and medicine in the US, the teaspoon is defined the same as a metric teaspoonprecisely 5 millilitres (mL). Dry ingredients For dry ingredients (e.g., salt, flour, spices), if a recipe calls for a level teaspoon, it refers to an approximately leveled filling of the spoon, producing the same volume as for liquids. A rounded teaspoon is a larger but less precise measure, produced by scooping a larger portion, then lightly tapping the spoon so that the loose ingredient falls away without leveling the ingredient off.* A heaping (North American English) or heaped (UK English) teaspoon is an even larger inexact measure consisting of the amount obtained by scooping the dry ingredient up as high as possible without leveling it off. For some ingredients, e.g. flour, this quantity can vary considerably. This is a clarification of the previous definition of "rounded" teaspoon, which was, in essence, identical to "heaping" teaspoon. Cite