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In heraldry, an annulet (i.e. "little ring") is a common charge. It may allude to the custom of prelates to receive their investiture per baculum et annulum ('by rod and ring'), and can also be described as a roundel that has been "voided" (i.e. with its centre cut out). In English and Canadian heraldry it is also used as the difference mark of a fifth son.
This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). "Annulet". Cyclopædia, or an Universal Dictionary of Arts and Sciences. 1 (1st ed.). James and John Knapton, et al. p. 104.