Cat brooch

The cat brooch or cat badge was one of several cat-shaped fashion accessories owned by the Sixth Doctor. He always wore one of his cat brooches on the left lapel of his coat.

The Doctor occasionally touched, tapped, or rubbed his cat brooch for good luck just before doing something particularly risky. (DW: The Twin Dilemma, Attack of the Cybermen, DWA Retribution, Time Wake)

When Peri Brown was put on trial for a murder she didn't commit, the Doctor proved her innocence in part by clever use of his cat brooch to collect a blood sample from the real killer. (IDW: The Forgotten)

One of the cat brooches appeared to change its expression to match the Doctor's mood. (WC: Real Time)

Behind the scenes

 * One original idea for the brooch was that it would change its expression to match the mood of the current situation; for example, frightened cats when the Doctor was scared. Eventually this idea was dropped, though it would be subtly implemented in the illustrated webcast Real Time.
 * The production team settled for using different cat brooches. Many fans sent cat brooches to Colin Baker; he eventually accumulated a large collection, and claims to have worn every single one. (DCOM: Attack of the Cybermen) Baker wore brooches from his collection as part of his costume during his theatrical run as the Doctor in Doctor Who: The Ultimate Adventure.