RKM code
The RKM code, also referred to as "letter and numeral code for resistance and capacitance values and tolerances", "letter and digit code for resistance and capacitance values and tolerances", or "R notation", is a notation to specify resistor and capacitor values defined in the international standard IEC 60062 (formerly IEC 62) since 1952. It is also adopted by various other standards including DIN 40825 (1973), BS 1852 (1974), IS 8186 (1976) and EN 60062 (1993). The significantly updated IEC 60062:2016 comprises the most recent release of the standard. Overview. Originally meant also as part marking code, this shorthand  notation is widely used in electrical engineering to denote the values of resistors and capacitors in circuit diagrams and in the production of electronic circuits (for example in bills of material and in silk screens). This method avoids overlooking the decimal separator, which may not be rendered reliably on components or when duplicating documents. The standards also define a "color code for fixed resistors". Part value code. For brevity, the notation omits to always specify the unit (ohm or farad) explicitly and instead relies on implicit knowledge raised from the usage of specific letters either only for resistors or for  capacitors, the case used (uppercase letters are typically used for resistors, lowercase letters for capacitors), a part's appearance, and the context. The notation also avoids using a decimal separator and replaces it by a letter associated with the prefix symbol for the particular value. This is not only for brevity (for example when printed on the part or PCB), but also to circumvent the problem that decimal separators tend to "disappear" when photocopying printed circuit diagrams. The code letters are loosely related to the corresponding SI prefix, but there are several exceptions, where the capitalization differs or alternative letters  are used. For example, codice_1 indicates a resistor value of 8.2 kΩ. Additional zeros imply tighter tolerance, for example codice_2. When the value can be expressed without the need for a prefix, an "R" is used instead of the decimal separator. For example, codice_3 indicates 1.2 Ω, and codice_4 indicates 18 Ω. For "resistances", the standard dictates the use of the uppercase letters codice_5 (for 10), codice_6 (for 10 = 1), codice_7 (for 10), codice_8 (for 10), and codice_9 (for 10) to be used instead of the decimal point. The usage of the letter codice_6 instead of the SI unit  symbol Ω for ohms stems from the fact that the Greek letter Ω is absent from most older character encodings (though it is present in the now-ubiquitous Unicode) and therefore is sometimes impossible to reproduce, in particular in some CAD/CAM environments. The letter codice_6 was chosen because visually it loosely resembles the Ω glyph, and also because it works nicely as a mnemonic for "r"esistance in many languages. The letters codice_9 and codice_13 weren't part of the first issue of the standard, which pre-dates the introduction of the SI system (hence the name "RKM code"), but  were added after the adoption of the corresponding SI prefixes. The introduction of the letter codice_5 in more recent issues of the standard (instead of an SI prefix codice_15 for milli) is justified to maintain the rule of only using uppercase letters for resistances (the otherwise resulting codice_8 was already in use for mega). Similar, the standard prescribes the following lowercase letters for "capacitances" to be used instead of the decimal point: codice_17 (for 10), codice_18 (for 10), codice_19 (for 10), codice_15 (for 10), but uppercase codice_21 (for 10 = 1) for farad. The letters codice_17 and codice_18 weren't  part of the first issue of the standard, but were added after the adoption of the corresponding SI prefixes. In cases where the Greek letter codice_19 is not available, the standard allows it to be replaced by codice_25 (or codice_26, when only uppercase letters are available). This usage of codice_25 instead of codice_19 is also in line with ISO 2955 (1974, 1983), DIN 66030 (Vornorm 1973; 1980, 2002) and BS 6430 (1983), which allow the prefix codice_29 to be substituted by the letter codice_25 (or codice_26) in circumstances in which only the Latin alphabet is available. Tolerance code. Letter code  for resistance and capacitance tolerances: Before the introduction of the RKM code, some of the letters for symmetrical tolerances (viz. G, J, K, M) were already used in US military contexts following the American War Standard (AWS) and Joint Army-Navy Specifications (JAN) since the mid-1940s. Temperature coefficient code. Letter codes for resistor temperature coefficients: Production date code. Example: V8 = August 2007 (or August 1987) Some manufacturers also used the production date code as a stand-alone code to indicate the production date of integrated circuits. Similar codes. A similar not standardized notation using the unit symbol instead of  a decimal separator is sometimes used to indicate voltages (3V3 for 3.3 V, or 1V8 for 1.8 V) in contexts where a decimal separator would be inappropriate (e.g. in signal names or file names).