In fax

Fax

Fax , sometimes called telecopying, is the telephonic transmission of scanned printed material , normally to a telephone number connected to a printer or other output device...

 systems, skew is the angular deviation of the received frame from rectangularity caused by asynchronism between the scanner

Image scanner

In computing, an image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. Common examples found in offices are variations of the desktop scanner where the document is placed on a glass...

 and the recorder. This is the same as the angle between the scanning line, or recording line, and the perpendicular

Perpendicular

In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...

 to the paper path.

Skew is expressed numerically as the tangent of the deviation angle.

Example of +1 degree (counter-clockwise) skew in a facsimile image. The red line is perfectly horizontal.