The Hipparcos Input Catalogue was constructed as the observing programme for the European Space Agency's Hipparcos astrometry mission. The requirements of the project in terms of completeness, sky coverage, astrometric and photometric accuracy, as well as the necessary optimisation of the scientific impact, resulted in an extended effort to compile and homogenize existing data, to clarify sources and identifications and, where needed, to collect new data matching the required accuracy.
This has resulted in an unprecedented catalogue of stellar data
including up-to-date information on positions, proper motions,
magnitudes and colours, and (whenever available) spectral types, radial
velocities, multiplicity and variability information. The catalogue is
complete to well-defined magnitude limits, and includes a substantial
sampling of the most important stellar categories present in the solar
neighbourhood beyond these limits. As explained below, the magnitude
limits vary from 7.3 to 9 mag as a function of galactic latitude and
spectral type, and there are no stars fainter than about 
mag.