What kind of shape does 'Tenkai' attached to the sake cup name mean?

'Tenkai (天開)' attached to the end of the sake cup's name refers to the cup's appearance. As the Chinese characters mean opening wide toward the top, Tenkai refers to a cup shape whose rim opens wider than its base. Toyo Sasaki's Basic Sake Cup Tenkai also has a shape where the circumference widens as it goes up from the base toward the rim^[1]. The total capacity held in one cup is 100ml^[2].

A cup with a rim flaring outward has the characteristic of allowing a wider contact area between the drink and lips and a gentle dispersion of aroma. However, as it is a thin glass cup, sudden temperature changes must be avoided when handling it. If you wrap the cup with a wet cloth or place it in a location with low temperatures while it is hot, the glass may break as it cools rapidly^[3].