Identifying a writer of a disputed signature or forgery, or of an anonymous writing utilizes a comparative examination of individualizing, writing characteristics - those writing habits that are unique to a certain writer. A combination of a sufficient number of similar characteristics and an absence of fundamental, unexplainable differences can lead to an opinion which identifies a writer.
Gathering sufficient comparison writing is crucial to aiding in an identification. Quality copies of known writing, and originals when they are available, are often ideal specimens for comparison which aid the examination. If an identification cannot be made, often an elimination - who did not author the disputed writing - can be made.