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The UBVRI photometric system, also called the Johnson-Cousins system, is a widely-used photometric system often generally assumed if no other system is specified or implied. It is an extension of the UBV photometric system (which defines the U, B, and V passbands), additionally including the (red) R band, and (infrared) I band. 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 uses the mean value of a number of stars over time, aiming for a more-consistent zero-magnitude that falls within Vega's magnitude-range.)