Pocket dimentions for Supers and Modern Horror. With some modification many are useable in other genres
Victorian Ladies used to use their decorative fans to communicate simple messages to their lovers while chaperones or husbands watched. These messages include "Yes", "No", "We've been discovered", "When can I meet you", etc. An Assassin's League of women could easily use this secret language to their advantage, sending messages to each other in hopes of reaching their target at a crowded ball.