These are indoor wall-mounted acrylic signs made of two sheets of acrylic sandwiched together by spacer studs. The message can be inserted between the sheets. Thus the message is protected from getting damaged. Usually both sheets of acrylic can be clear or the bottom sheet can be opaque also while the top sheet will be clear.

Advantages : The message is protected from damage, cleaner look

Disadvantages : Can be tampered with easily.