Star numbering is probably the most serious problem to solve to build a coherent database. The solution adopted is based on earlier work which was undertaken to include photometric data on star clusters into catalogues prepared for the Strasbourg Data Center (Mermilliod 1972, 1973, 1976b, 1979a).
One numbering system has been adopted for each cluster and it provides a unique identificator for each star which is used to register the various data. Therefore the data recorded under the numer #1 in each data file always belong to the same star, i.e. star #1. This identificator is the main key that will be used to access the data. This policy implies that star original numbers found in publications have often to be transformed into the system adopted in the database. Complete cross-references have been determined, either by comparing cluster charts or by using rectangular (x,y) positions and carefully kept in tables, one per cluster. These tables give, for each star, the different identification numbers that are existing in the various numbering systems. The first column of the cross-reference table defines the adopted numbering system. The second column of the table contains the numbers from the basic source and the numbers adopted in BDA are therefore identical to it. But no numbering system contains all the stars present in the cluster field and the basic numbering systems have been extended.
Two solutions have been adopted to record additional stars. The first one consists in continuing the numering system with the new stars of the second reference, entered in increasing number order, then with the third system and so on. This works fine if the additional stars are not too numerous. The second solution is used when one numbering system contains many new stars, but it is not desirable to change the reference system. This happens, for example, when proper motion studies contain lots of stars outside the main cluster area. In these cases, a constant is added to the star numbers. Adding 1000, makes it easy to read back the original numbers.