Text in force at 23-63: from 23-50, effective 11 Mar 1996.
For each instrument--
(a) When markings are on the cover glass of the instrument, there must be means to maintain the correct alignment of the glass cover with the face of the dial; and
(b) Each arc and line must be wide enough and located to be clearly visible to the pilot.
[(c) All related instruments must be calibrated in compatible units.]
61 FR 5192 · Source