Vintage Ships Radio Room Wall Clock with emergency silence period markings. Great for your Hamshack! The sinking of the Titanic resulted in the Radio Act of 1912, requiring 24-hour radio watches at sea. The disaster also led to this clock design, which features two 3-minute periods marked in red, indicating Morse Code silence periods when only distress, urgent, and safety signals could be transmitted.Two green markings, likewise, designate silent periods on voice transmissions, where one would listen for or transmit distress signals. All ships and coastal stations kept a mandatory listening watch for such signals. This retro clock isn’t just ridiculously good looking. The marking are actually very useful for ham radio operating. They can remind you to listen for your ham friends at pre determined frequencies at the top and bottom of the hour for voice with the green marking. Then to try to make contact using CW Morse code during the red periods if the band conditions are too poor for voice. This clock has a keyhole at the back for easy hanging on a wall. They're made from an acrylic material that looks like glass but is more durable and sturdy. .: Material: 100% acrylic .: Two size options (10.75''×10.75'' & 8''×8'') .: Requires one AA battery (NOT included) .: Includes keyhole hanging slot