Compound Indexes
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     A compound index is an index file that contains multiple indexes (called
     tags).  Compound indexes (.cdx)'s make several indexes available to your
     application while only using one file handle.  Therefore, you can
     overcome the CA-Clipper index file limit of 15. A compound index can
     have as many as 99 tags, but the practical limit is around 50.  Once you
     open a compound index, all the tags in the file are automatically
     updated as the records are changed.

     Once you open a compound index, all the tags contained in the file are
     automatically updated as the records are changed.  A tag in a compound
     index is essentially identical to an individual index (.idx) and
     supports all the same features.  The first tag (in order of creation) in
     the compound index is, by default, the controlling index.