b0

B0 is a designation that appears in several different fields, where it denotes distinct concepts rather than a single object. Its meaning depends on the context, and compared to other “B” labels it is not part of a unified system.

In music, B0 refers to the note B in the first octave (octave 0). It is the lowest B on a standard 88‑key piano, with a frequency of about 30.87 Hz. This note sits at the bottom end of the piano’s range and is part of the standard pitch nomenclature used in Western music.

In printing and paper sizes, B0 is an ISO 216 paper size in the B series. It measures 1000 by 1414 millimeters (approximately 39.37 by 55.67 inches). The B-series sizes use the same aspect ratio as the A-series and are designed to provide intermediate options between A sizes, with B0 covering roughly 1.414 square meters of area.

In astronomy, B0 denotes a hot, blue-white stellar spectral type. B0 stars are among the hotter stars in the Morgan–Keenan system, with surface temperatures around 30,000 Kelvin. They are typically luminous, massive, and often appear blue in color; many are found on or near the main sequence and can be prominent in young star clusters.

In physics, B0 commonly appears as a symbol for a reference or initial magnetic flux density in equations and models. It is used in various subfields, such as magnetism, superconductivity, and magnetic resonance, to denote a baseline or starting field value, distinct from fields at other times or conditions.