Text in force at CS27-10: from CS27-0, effective 14 Nov 2003.
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.
ED Decision 2003/015/RM · Source