The volumes of indexes are usually subdivided into parts, including lists in number order and lists divided in subject order. There is extensive top numbering present amongst the drawings and the indexes often have lists enabling comparison of the drawing numbers. Amongst the ZB class locomotives there are, for instance, the GEC Australia drawing numbers that have an AB (Australian built) prefix, an alpha numeric drawing number with DEU prefix that was used by the Queensland Railways and additional drawings by both the Queensland and Tasmanian railways mechanical engineering sections that are indexed amongst their main drawing series but have been included in these indexes. In some cases the index runs into several volumes for a single class of locomotive.