I would like to know whether the official english SUS questions are copyrighted, or if I may use them in a commercial product.
A Google Search doesn't provide many hints, although the paper in which the questions are presented is copyrighted.
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The paper "SUS - A quick and dirty usability scale" has a copyright notice in it: "© Digital Equipment Corporation, 1986." This copyright notice is shown on exactly those pages where the SUS scale itself is presented; to me, that indicates that the copyright notice is intended to apply to the scale itself, rather than the paper as a whole.
However, the paper also states:
SUS has been made freely available for use in usability assessment, and has been used for a variety of research projects and industrial evaluations; the only prerequisite for its use is that any published report should acknowledge the source of the measure.
This seems to constitute a statement of permission to use the scale under certain conditions.