On May 1, 1911, the new states' postal rates became unified due to the expansion of the United Kingdom's domestic postal rate of 1d per half ounce (Imperial Penny Post) to Australia as a part of the British Empire. The universal domestic mail rate became one penny. In 1911, one-cent postcards and lettercards were also available. These items are rare but may be found in larger cities like Sydney or Melbourne. The one-cent stamp was issued in sheets of 12 stamps each with eight different subjects. These stamps were used until 1935 when they were all overprinted 10d stamps. The one-cent stamp was issued in two varieties: flat and embossed. Flat one-cent stamps were used from 1911 to 1917 when they were replaced by embossed ones. Flat one-cent stamps were printed in four colors on paper that was later used for printing postage stamps up to 1965. From 1917 on, only three colors were used - green, black and yellow - on special paper called "Waterproof Paper." This made the one-cent stamp more durable and allowed them to be used for mailing letters, packages and postcards under water. Embossed one-cent stamps were issued from 1918 to 1935. They are much harder to find than their flat counterparts because very few people knew they existed until now. There are three types of embossed one-cent stamps: regular, souvenir and hunter's sets.