Little yellow map pin
Traditionally, a little yellow map pin (sometimes abbreviated "LYMP"), worn in a member's lapel, was the "secret" symbol of Mensa membership intended to enable Mensans to recognize one another upon meeting by chance. For many years, such a pin was included in the new-member packet sent to those joining American Mensa, and perhaps some other national Mensas did it as well. However, the custom of wearing them gradually went into disuse.
Various hearsay rumors have spread to explain why the pin is no longer sent to new members. A Creative Mischief SIG humorous sign once claimed that due to liability risks of injuries from map pins, they were being replaced with diaper pins. An anecdote also held that the use of the pins was discontinued because they were too similar to pins used to identify Secret Service agents guarding the President of the United States; supposedly, in the 1970s or 1980s, former American Mensa chairman Marvin Grosswirth was arrested for wearing his map pin near the United Nations. However, a more prosaic explanation is that the use of pins went into decline and was eventually eliminated because fewer people wear suits with lapels these days.