PART IV
SERIES B AND C RECOMMENDATIONS
SERIES B RECOMMENDATIONS
Means of expression
Recommendation
	No.									Title
	B.1	Letter symbols for telecommunications
	B.3	Use of the international system of units (SI)
	B.10	Graphical symbols and rules for the preparation of documentation in telecommunications
	B.11	Legal time; use of the term UTC
	B.12	Use of the decibel and the neper
	B.13	Terms and definitions
	B.14	Terms and symbols for information quantities in telecommunications
	B.15	Nomenclature of the frequency and wavelength bands used in telecommunications
	B.16	Use of certain terms linked with physical quantities
	B.17	Adoption of the CCITT Specification and Description Language (SDL)
	B.18	Traffic intensity unit
	B.19	Abbreviations and initials used in telecommunications
Recommendation B.11) 
LETTER SYMBOLS FOR TELECOMMUNICATIONS
	The CCITT,
considering
	(a)	that in order to simplify the reading of documents dealing with telecommunication technique, it is essential to use simple notations in a homogeneous system and having well–defined meaning; that, moreover, it is an advantage, wherever possible, to have notations that have been universally adopted;
	(b)	that CMV is collaborating with Technical Committee No. 25 of the IEC,
recommends
	that in their mutual relations the ITU and its permanent organs and Administrations and recognized private operating agencies use in all languages, wherever possible, the letter symbols and the notations recommended by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) to represent the physical quantities and the mathematical operations.
	Note – References of relevant publications (updated in 1988).
	ISO Standard 31: “General principles concerning quantities, units and symbols”
	Parts of ISO Standard 31 of greatest interest for telecommunications:
0		(General principles)
I		(Quantities and units of space and time)
II		(Quantities and units of periodic and related phenomena)
V		(Quantities and units of electricity and magnetism)
VI		(Quantities and units of light and related electromagnetic radiations)
VII		(Quantities and units of acoustics)
XI		(Mathematical signs and symbols)
	IEC Publication 27: “Letter symbols to be used in electrical technology”
	27–1	General 	(1971 Edit. 5, with 1974 Amend. 1 and 1977 Amend. 2) (1983 Amend. 4, including 1981 Amend. 3)
	27–1A	(1976)		(Time–dependent quantities)
	27–2	(1972)		(Telecommunications and electronics)
	27–2A	(1975)		(First supplement: waveguide propagation; scattering matrix and transfer matrix; static convectors; automatic control science and technology)
	27–2B	(1980)		(Second supplement: linear n–port networks)
	27–3	(1974)		(Logarithmic quantities and units) (see CCIR Recommendation 574)
	27–4	(1985)		(Quantities to be used for rotating electric machines)
	See also IEC Handbook Letter Symbols (1983) and the Directives applicable to the work of the IEC on letter Symbols (1986).