Before Present (BP) years is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred prior to the origin of practical radiocarbon dating in the 1950s. Because the "present" time changes, standard practice is to use 1 January 1950 as the commencement date (epoch) of the age scale.[a] The abbreviation "BP" has been interpreted retrospectively as "Before Physics"; that refers to the time before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, making dating after that time likely to be unreliable.
In a convention that is not always observed, many sources restrict the use of BP dates to those produced with radiocarbon dating; the alternative notation RCYBP is explicitly Radio Carbon Years Before Present.
The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy. This usage differs with the recommendation by van der Plicht & Hogg, followed by the Quaternary Science Reviews, both of which requested that publications should use the unit "a" for year and reserve the term "BP" for radiocarbon estimations.
Radiocarbon dating was first used in 1940. Beginning in 1954, metrologists established 1950 as the origin year for the BP scale for use with radiocarbon dating, using a 1950-based reference sample of oxalic acid. According to scientist A. Currie Lloyd:
The problem was tackled by the international radiocarbon community in the late 1950s, in cooperation with the U.S. National Bureau of Standards. A large quantity of contemporary oxalic acid dihydrate was prepared as NBS Standard Reference Material (SRM) 4990B. Its 14C concentration was about 5% above what was believed to be the natural level, so the standard for radiocarbon dating was defined as 0.95 times the 14C concentration of this material, adjusted to a 13C reference value of −19 per mil (PDB). This value is defined as "modern carbon" referenced to AD 1950. Radiocarbon measurements are compared to this modern carbon value, and expressed as "fraction of modern" (fM). "Radiocarbon ages" are calculated from fM using the exponential decay relation and the "Libby half-life" 5568 a. The ages are expressed in years before present (BP) where "present" is defined as AD 1950.
The year 1950 was chosen because it was the standard astronomical epoch at that time. It also marked  the publication of the first radiocarbon dates in December 1949, and 1950 also antedates large scale atmospheric testing of nuclear weapons, which altered the global ratio of carbon-14 to carbon-12.
Dates determined using radiocarbon dating come as two kinds: uncalibrated (also called Libby or raw) and calibrated (also called Cambridge) dates. Uncalibrated radiocarbon dates should be clearly noted as such by "uncalibrated years BP", because they are not identical to calendar dates. This has to do with the fact that the level of atmospheric radiocarbon (carbon-14 or 14C) has not been strictly constant during the span of time that can be radiocarbon-dated. Uncalibrated radiocarbon ages can be converted to calendar dates by means of calibration curves based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as dendrochronology (dating on the basis of tree growth-rings) and stratigraphy (dating on the basis of sediment layers in mud or sedimentary rock). Such calibrated dates are expressed as cal BP, where "cal" indicates "calibrated years", or "calendar years", before 1950.
Many scholarly/scientific journals require that published calibrated results be accompanied by the name (standard codes are used) of the laboratory concerned, and other information such as confidence levels, because of differences between the methods used by different laboratories and changes in calibrating methods.