until version 6.0, "pre-image paging" was used when performing updates to records. it involved the creation of a new "pre-image file" before the changes were made, and then the pages from the original data file were copied into this new pre-image file temporarily. then the system would make the changes to the original file. should the update be interrupted and only half the data written to the page, then the page would just be rolled back by the engine by copying the page from the pre-image file back into the corrupted page in the original database file, then the temporary pre-image file would be deleted. the preimage files were given the extension .pre, so finding these files in the system usually indicated that a transaction had not happened correctly and recovery had not been successful.