Latin letters except for the symbol for micro, which is uniquely a Greek letter "". Like the numbers they combine with, SI units and unit symbols are never shown in italics. The prefixes and their symbols are always prefixed to the symbol for the unit without any intervening space or punctuation. This distinguishes a prefixed unit symbol from the product of unit symbols, for which a space or mid-height dot as separator is required. So, for instance, while 'ms' means millisecond, 'm s' or 'm·s' means metre second. Prefixes corresponding to an integer power of one thousand are generally preferred, and the prefixes for tens (deci-, deca-) and hundreds (cent-, hecto-) are disfavoured. Hence 100 m is preferred over 1 hm (hectometre) or 10 dam (decametres). The prefixes deci- and centi-, and less frequently hecto- and deca-, are commonly used for everyday purposes; the centimetre (cm) is especially common. Some modern building codes require that the millimetre be used in preference to the centimetre, because "use of centimetres leads to extensive usage of decimal points and confusion". Deprecated prefixes are also used to create metric units corresponding to older conventional units, for example hectares and hectopascals. Prefixes may not be used in combination on a single symbol. This includes the case of the base unit kilogram, which already contains a prefix. For example, milligram (mg) is used instead of microkilogram (μkg). In the arithmetic of measurements having units, the units are treated as multiplicative factors to values. In the product of multiple units, each individual unit prefix must be evaluated as a separate numeric multiplier and then combined with the others. A prefix symbol attached to a unit symbol is included when the unit is raised to a power. For example, km2 is km × km, not km × m. Usage Examples The mass of an electron is about 1 rg (rontogram). The mass of 1 litre of water is about 1 kg (kilogram). The mass of the Earth is about 6 Rg (ronnagrams). The mass of Jupiter is about 2 Qg (quettagrams). Examples of powers of units with metric prefixes 1 km2 means one square kilometre, or the area of a square of by . In other words, an area of square metres and not square metres. 2 Mm3 means two cubic megametres, or the volume of two cubes of by by or , and not cubic metres ().