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A bustline is an arbitrary line encircling the fullest part of the bust or body circumference at the bust. It is a body measurement which measures the circumference of a woman's torso at the level of the breasts. It is measured by keeping a measuring tape horizontal and wrapping it around the body so that it goes over the nipples and under the arms.
In relation to a woman's garment, the bustline is the outline or shape of a woman's bust, or the part of the garment which covers the breasts, such as a dress with a fitted bustline. The bustline is an important measure in women's clothing sizes.
A full bust is another term for the bustline measure but is also commonly used to refer to a breast with a cup size of at least C.
A high bust is a measure of a woman's torso above her breasts.
An under-bust is the measure below the breasts.