This system was highly optimized for classification of stars from ground-based observations.
The system was chosen to be medium-band, to ensure the possibility to measure faint stars.
The Y bandpass is near the breakpoint of the interstellar extinction law (interstellar extinction in the 300–800 nm region can be approximated by two straight lines, which intersect at ~435.5 nm).
The P magnitude is placed exactly on the Balmer jump in order to provide separation for luminosity classes of B-A-F stars.
Finally, the V magnitude is chosen to coincide with a similar bandpass in the UBV system.
The following table shows the characteristics of each of the filters used (represented colors are only approximate): UBV photometric system