The 6¢ denomination covered the domestic rate for mail weighing up to three ounces prior to U.S. entry into World War I. When the new wartime rate for domestic first rate mail increased from 2¢ to 3¢, U.S. #506 fulfilled the postage for letters weighing two to three ounces. Large quantities were issued – greater than all previous 6¢ denominated stamps combined. This was due to the high demand for stamps brought on by World War I and the rate change.