A placard is placed on a building to indicate special information about that building. The most common placard which had been used is the Fallout shelter black and yellow trefoil indicating that a building was specifically prepared for use for emergency shelter in the event of a nuclear explosion in the area of the particular building. Temporary placards may be placed on buildings such as warning signs over tenting to indicate the building is being fumigated and that no one should enter.
In some locations it is common practice, and in some areas (such as California) it is required by law that any primary entrance to a commercial building have a placard attached above the entrance door(s) reading "This door to remain unlocked during business hours.
Commercial vehicle placards are of two general types, the single indicator, where in specific information of a fixed nature is displayed by the sign, such as the yellow "oxidizing agent" sign below, and the NFPA "fire diamond" style device which can be changed to depict new information as the contents of the vehicle change, and which carries information about the compound being carried, its flamability and resistance to water, for example.
|td>||This is an example of a fixed sign.||This is another example of a fixed sign, although the information is basically for the use of emergency responders.||This is an example of a changeable sign. This type of sign is sometimes displayed as a fixed sticker, but are often made with each of the four quadrants having either a slot to have different values inserted, or have a flip device allowing each of the quadrants to be changed.|