the variety of English used. US English uses the spelling deka-, meter, and liter, whilst International English uses deca-, metre, and litre. Additionally, the name of the unit whose symbol is t and which is defined according to is 'metric ton' US English but 'tonne' in International English. Unit symbols and the values of quantities Symbols of SI units are intended to be unique and universal, independent of the context language. The SI Brochure has specific rules for writing them. The guideline produced by the National Institute of Standards and Technology (NIST) clarifies language-specific details for American English that were left unclear by the SI Brochure, but is otherwise identical to the SI Brochure. General rules General rules for writing SI units and quantities apply to text that is either handwritten or produced using an automated process: The value of a quantity is written as a number followed by a space (representing a multiplication sign) and a unit symbol; e.g., 2.21 kg, , 22 K. This rule explicitly includes the percent sign (%) and the symbol for degrees Celsius (°C). Exceptions are the symbols for plane angular degrees, minutes, and seconds (°, ′, and ″, respectively), which are placed immediately after the number with no intervening space. Symbols are mathematical entities, not abbreviations, and as such do not have an appended period/full stop (.), unless the rules of grammar demand one for another reason, such as denoting the end of a sentence. A prefix is part of the unit, and its symbol is prepended to a unit symbol without a separator (e.g., k in km, M in MPa, G in GHz, μ in μg). Compound prefixes are not allowed. A prefixed unit is atomic in expressions (e.g., km2 is equivalent to (km)2). Unit symbols are written using roman (upright) type, regardless of the type used in the surrounding text. Symbols for derived units formed by multiplication are joined with a centre dot (⋅) or a non-breaking space; e.g., N⋅m or N m. Symbols for derived units formed by division are joined with a solidus (/), or given as a negative exponent. E.g., the "metre per second" can be written m/s, m s−1, m⋅s−1, or . In cases where a solidus is followed by a centre dot (or space), or more than one solidus is present, parentheses must be used to avoid ambiguity; e.g., kg/(m⋅s2), kg⋅m−1⋅s−2, and (kg/m)/s2 are acceptable, but