The Danish bi-colored stamps (1875 - 1905) are wonderful stamps. Pretty and full of unsolved mysteries. You can spend hours learning about how they were designed, produced and finally delivered by the printers to the Postal Authorities. If you have studied enough, which presupposes having enough material; If you have acces to the right literature and have acquired some experience in identifying the secreets, you may be able to determine a stamp by its printing and maybe even by its original position in the sheet.
The Danish bi-colored stamps are not rare stamps. Most of them were produced in millions. Some of the denominations you can even still buy by the hundreds for 15 - 20 $. Others will cost you more.
Their design consists of a frame and an oval. In order to determine their type according to the catalogues you need to understand at least the difference between a normal and an inverted frame. You will see the difference clearly from the old drawing shown above.
I will not dwell on the details of the differences between the two types in this posting. The point I want to make is, that you can only very rarely determine their number in the catalog and thereby their value by their type of frame alone. You need to combine the information of the type with more information to do that. Therefore never accept information given at auction or by a seller about the type of a stamp as the only and final proof of the identity of a stamp if the auctioneer or the seller claims that the stamp is rare and valuable because of its type of frame alone.
Some stamps with a normal frame can in facty be very rare; some stamps with an inverted frame can also in fact be very rare. It all depends, which denomination and which printing they belong to. And in order to determine that, you need a whole lot more information about this series and experience in dealing with this information as I have already stated above.Therefore if tempted by an offer of a bi-cored stamp with a rare frame always demand a certificate by an expert known to posses the necessary knowledge about the subject.
Here follows two samples. One stamp with an inverted frame and one stamp with a normal frame. Soon I'll come back to complicate matters telling about thick frames, which can also be devided into the to types mentioned.