Sister Young told of a meeting held in the Logan Temple where the late Pres[ident] [Anthony W.] Ivins stated that the full meaning of the marks of the garment has not been revealed. All garments worn must have the approved mark. When receiving your endowments the garment should not be placed on the individual until after it is marked. All ordinances should be given with closed eyes.
[Source: Letter from First Presidency, Manti Temple Historical Record, May 1, 1938, in Anderson, Devery; The Development of LDS Temple Worship, 1846-2000: A Documentary History, http://amzn.to/TempleWorship]