*Notation*

Upper and lower case letters are used to distinguish between quantities, which will never be determined exactly, and values that will be known (such as a numerical indication on a measuring instrument), respectively. For example, a measurement result *y* is written in lower-case because a result always has a definite numerical value: The value *y* is an estimate of *Y*, which is the quantity intended to be measured. Upper-case is used for *Y* to indicate that the quantity cannot be known exactly. It is helpful to make this distinction because uncertain-number objects are used to represent quantities; the associated numerical estimates and other known values, such as uncertainty, will appear as attributes of uncertain numbers.
