Quote from the PDF linked below (page 26) in the table titled "Table 3. Coherent derived units in the SI with special names and symbols" about the 'SI coherent derived unit' for the 'plane angle' unit of radian:
{{Quote|text=<blockquote>(b) The radian and steradian are special names for the number one that may be used to convey information about the quantity concerned. In practice the symbols rad and sr are used where appropriate, but the symbol for the derived unit one is generally omitted in specifying the values of dimensionless quantities.|sign=Organisation Intergouvernementale de la Convention du Mètre|source=[http:<//www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf The International System of Units(SI)], page 26}}blockquote>
Quote from the PDF linked below (page 28) in the section titled "2.2.3 Units for dimensionless quantities, also called quantities of dimension one":
"<blockquote>In a few cases, however, a special name is given to the unit one, in order to facilitate the identification of the quantity involved. This is the case for the radian and the steradian. The radian and steradian have been identified by the CGPM as special names for the coherent derived unit one, to be used to express values of plane angle and solid angle, respectively, and are therefore included in Table 3."</blockquote>
Quote from the PDF linked below (page 42) in the section titled "5.3.7 Stating values of dimensionless quantities, or quantities of dimension one". This is not very clear but explains why the rad unit is many times hidden, and why the other dimensionless units such as % and ppm must be stated (need to read the whole section for that):
"<blockquote>As discussed in Section 2.2.3, the coherent SI unit for dimensionless quantities, also termed quantities of dimension one, is the number one, symbol 1. Values of such quantities are expressed simply as numbers. The unit symbol 1 or unit name "one" are not explicitly shown, nor are special symbols or names given to the unit one, apart from a few exceptions as follows. For the quantity plane angle, the unit one is given the special name radian, symbol rad, and for the quantity solid angle, the unit one is given the special name steradian, symbol sr. For the logarithmic ratio quantities, the special names neper, symbol Np, bel, symbol B, and decibel, symbol dB, are used (see 4.1 and Table 8, p. 127).</blockquote> Quotes from the PDF linked below (page 67) from the appendix section titled "SI supplementary units (radian and steradian)": <blockquote>...the units radian and steradian are usually introduced into expressions for units when there is need for clarification...</blockquote> Quote from the PDF linked below (page 67) from the appendix section titled "Elimination of the class of supplementary units in the SI" for resolution 8 of the CGPM conference: <blockquote>decides...</blockquote>
Quotes from <blockquote>to interpret the PDF linked below (page 67) from supplementary units in the appendix sectiontitled "SI supplementary units (, namely the radian and the steradian)":"..., as dimensionless derived units, the units radian names and steradian are usually introduced intosymbols of which may, but need not, be used in expressions for other SI derived units when there , as is need for clarification..."convenient,</blockquote>
Quote from the PDF linked below (page 67) from the appendix sectiontitled "Elimination of the class of supplementary units in the SI" forresolution 8 of the CGPM conference:"decides...""to interpret the supplementary units in the SI, namely the radian andthe steradian, as dimensionless derived units, the names and symbolsof which may, but need not, be used in expressions for other SIderived units, as is convenient,""<blockquote>and, consequently, to eliminate the class of supplementary units as aseparate class in the SI."</blockquote>
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