Before the advent of the Giori Press, which allowed easy multicolor printing, the United States Post Office issued primarily single colored stamps. This was because bi color stamps, under the traditional engraving process, required different runs through the press for each color. This was time consuming and created “registration” problems, that is problems of the colors aligning correctly one to the other. But other, more disastrous, problems, such as inverts, could occur. To 1960, the United States issued less than 20 bi colored stamps. The first were the higher values of the 1869 with two of the three values being known inverted. The next issue was 32 years later, the 1901 Pan American issue and two of these values are known inverted. And of course there is the famous 24c Airmail invert. I have never seen what would be a wonderful thematic collection-bi colored engraved US stamps. First of all, all of the stamps are affordable (without the inverts) and so is completable, the possibility of which is important for any collection. Second, the stamps themselves are all among the nicest ever produced by the United States. Third, the stamps span a hundred year period that is very interesting from an historical and postal historical point of view, making collecting these stamps as used on cover fascinating. And last, these stamps produced an enormous number of varieties that are unique to bi colored printing-especially varieties of placement of the center to the frame, siderographer markings, and various shifts. A specialty that combines the technical aspects of our hobby with a broad historical period, and with affordability is well worth considering.