A variety of methods have traditionally been used to remove skew by mechanical means. Most document feeders incorporate some of these methods. These mechanical methods have a long history, many of them having originated in the printing equipment industry. The length of a document along the direction of travel has an impact on its tendency to skew and on the effects of any skew that does occur.
Longer documents are typically less inclined to skew.
In the feeder, this is because a longer document has a wider base for the alignment operation, whether this is performed by a person at load time or by drive rollers at feed time. A longer distance better assures the parallelism of the document edge with the feeder guides. In the transport, a longer document is typically held by more rollers or belts simultaneously, making it harder for it to rotate. Unfortunately, despite the lower likelihood of skew occurring, if a given skew angle is introduced into a longer document, more translation of the document edge across the direction of travel will occur than for a shorter document. In the extreme case, a trailing corner of the document may actually leave the camera's field of view if a generous overscan is not provided.