A mirror is a polished pane of metal or glass, set in a frame, and used to look at oneself. The standard heraldic form is more fully termed a “hand mirror”: shown in later period with a handle attached to the frame, as in the arms of Sybell, 1531 [Gwynn-Jones 69], but earlier as a simple circular framed mirror, as in the canting arms (German Spiegel) of Spiegel c.1370 [Gelre 29v]. In either of these forms, the mirror is the traditional accoutrement of the mermaid.
Period armory also has examples of “wall mirrors”, mounted in large rectangular frames, as in the arms of von Steuben or Stuben c.1450 [Ingeram 136, also Siebmacher 117].
The Order of the Mermaids Pearl of Trimaris bears: Argent, a handmirror azure silvered argent.
Branwen MacRae bears: Argent, a handmirror bendwise sinister sable, issuant from the glass tongues of flame proper.
Ailís inghean Mhuirgein bears: Vert, on a mirror argent glassed vert, an oak leaf argent.