kg/m/s2 and kg/m⋅s2 are ambiguous and unacceptable. The first letter of symbols for units derived from the name of a person is written in upper case; otherwise, they are written in lower case. E.g., the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa", but the symbol for mole is written "mol". Thus, "T" is the symbol for tesla, a measure of magnetic field strength, and "t" the symbol for tonne, a measure of mass. Since 1979, the litre may exceptionally be written using either an uppercase "L" or a lowercase "l", a decision prompted by the similarity of the lowercase letter "l" to the numeral "1", especially with certain typefaces or English-style handwriting. The American NIST recommends that within the United States "L" be used rather than "l". Symbols do not have a plural form, e.g., 25 kg, not 25 . Uppercase and lowercase prefixes are not interchangeable. E.g., the quantities 1 mW and 1 MW represent two different quantities (milliwatt and megawatt). The symbol for the decimal marker is either a point or comma on the line. In practice, the decimal point is used in most English-speaking countries and most of Asia, and the comma in most of Latin America and in continental European countries. Spaces should be used as a thousands separator () in contrast to commas or periods (1,000,000 or 1.000.000) to reduce confusion resulting from the variation between these forms in different countries. Any line-break inside a number, inside a compound unit, or between number and unit should be avoided. Where this is not possible, line breaks should coincide with thousands separators. Because the value of "billion" and "trillion" varies between languages, the dimensionless terms "ppb" (parts per billion) and "ppt" (parts per trillion) should be avoided. The SI Brochure does not suggest alternatives. Printing SI symbols The rules covering printing of quantities and units are part of ISO 80000-1:2009. Further rules are specified in respect of production of text using printing presses, word processors, typewriters, and the like. International System of Quantities SI Brochure The CGPM publishes a brochure that defines and presents the SI. Its official version is in French, in line with the Metre Convention. It leaves some scope for local variations, particularly regarding unit names and terms in different languages. The writing and maintenance of the CGPM brochure is carried out by one of the committees