The database files created by MUPIP CREATE are stored on random access (disk) media as RMS files recognized in the operating system environment by their .DAT extensions They have a proprietary internal structure defined as the Greystone Database Structure (GDS). These files are alternately referred to as either GDS files or simply database files. Both terms describe the database files that house global variable and lock resource names. This chapter describes the internal structure of GDS files. It assumes you are acquainted with the concepts of Balanced trees (B-trees).