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The UBV photometric system, also called the Johnson system or Johnson-Morgan system, is an early-adopted, widely-used photometric system and is a system generally assumed if no other system is specified or implied. It specifies U, B, and V passbands, which in turn specifying B-V and U-B color indices. The magnitude calibration is (traditionally) chosen to give Vega's apparent magnitude zero, i.e., the Vega system of calibration, so that the B-V and U-B color indexes are zero for stars of Vega's spectral type, i.e., A0 V stars (given compensation for reddening). (However, modern equipment senses Vega's small magnitude variations, and today's Vega system is determined using a number of stars, aiming to yield a more-consistent zero-magnitude that falls within Vega's magnitude range.)
The UBVRI photometric system, i.e., Johnson-Cousins system is an extension with red and infrared passbands, R band and I band.