The North polar sequence is a group of 96 stars that was used to define stellar magnitudes and colors. The cluster of stars lies within two degrees of the Northern Celestial pole, which makes them visible to everyone on the northern hemisphere.
Originally proposed by Edward Charles Pickering, the system was used between 1900 and 1950. Today it has been replaced by the UBV photometric system.
References
North polar sequence Wikipedia(Text) CC BY-SA