When something is written on a document which is on top of other documents, impressions of that writing may be transmitted through to the lower documents. Thus, any document could contain indented impressions from handwritten details on other documents.
These impressions of writing can be visualised using the Electrostatic Detection Apparatus (ESDA), which develops an accurate image of them. This process is repeatable and non-destructive, and is extremely sensitive. It can be used to detect impressions which are not visible to the naked eye.
This type of examination can be used in a variety of cases. For example, it can be used for intelligence purposes to determine the source of a document and may provide information as to the document’s history; to detect handwriting for further comparisons; to determine whether pages have been removed from a notebook or inserted later; or to determine whether entries in a book have been written in chronological order.