(including planetary sciences and solar physics) references, physics (including instrumentation and geosciences) references, as well as preprints of scientific papers from arXiv. The astronomy database is by far the most advanced and its use accounts for about 85% of the total ADS usage. Articles are assigned to the different databases according to the subject rather than the journal they are published in, so that articles from any one journal might appear in all three subject databases. The separation of the databases allows searching in each discipline to be tailored, so that words can automatically be given different weight functions in different database searches, depending on how common they are in the relevant field.