an index to a classification scheme, in which entries are generated by successive left truncation of strings of terms representing compound concepts
e.g. in the example of citation order, a compound concept is represented by the pre-coordinated string
bicycles - tyres - punctured - repairing - instruction books
In a classification scheme arranged in this way, everything on bicycles will be grouped together, but material on tyres or instruction books will be scattered. To provide index entries to allow these scattered topics to be found, we write the string in the reverse order, and successively truncate it from the left, making an index entry for each resulting substring:
instruction books - repairing - punctured tyres - bicyclesThese entries are then arranged in alphabetical order:
bicyclesEach of these index entries would be followed by the appropriate notation to link it to its place in the classification scheme. As the citation order of this classification determines that everything about tyres for bicycles will be grouped together in the classified sequence, it is not necessary for the index to have entries such as tyres - bicycles - punctured, or other combinations and permutations of the terms in the string. A chain index is thus more economical than a fully permuted index, in which a string of five terms would generate 120 index entries.
The mechanical method of generating a chain index described here may be modified by editorial intervention to suppress entries which are likely to be unsought, and to combine terms grammatically to make the index entries more readable; this has been done in the above example where punctured tyres has been used rather than punctured - tyres.