The system for mail delivery in the United States has developed with the nation. Rates were based on the distance between sender and receiver in the early years of the nation. In the middle of the 19th century, rates stabilized to one price regardless of distance. Rates were relatively unchanged until 1968, when the price was increased every few years by a small amount. Comparing the increases with a price index, the price of a first class stamp has been steady. The logo for the Post Office showed a man on a running horse, even as the railroads and then motorized trucks moved mail. In 1970, the Post Office became the Postal Service, with rates set by the Postal Regulatory Commission, and some oversight by the Congress. Air mail became standard in 1975. In the 21st century, prices were segmented to match the sorting machinery in use; letters too large for the machines required slightly higher postage. Postal rates to 1847 Initial United States postage rates were set by Congress as part of the Postal Service Act signed into law by President George Washington on February 20, 1792. The postal rate varied according to "distance zone", the distance a letter was to be carried from the post office where it entered the mail to its final destination. Rates were adopted in 1847 for mail to or from the Pacific Coast and in 1848 for mail sent from one place in the west to another place in the west. There were double and triple rates as a letter's size increased. There were ship fees which were also added (i.e. mail to Hawaii). The ship fee, including the ship rate on letters for delivery at the port of entry, were on a per letter basis, rather than weight. The United States issued its first postage stamps in 1847. Before that time, the rates, dates and origin of the letter were written by hand or sometimes in combination with a handstamp device. US Postal Service Since enactment of the Postal Reorganization Act of 1970, postage rates have been set by the Postal Regulatory Commission. Historical rates Historical trend Taking the above data and plotting it yields the graph shown to the right. The dark plot is the actual issued price of the stamp and the light plot is the price adjusted for inflation and is shown in 2019 US cents. This plot shows