Because governments issue stamps of different denominations in unequal numbers and routinely discontinue some lines and introduce others, and because of their illustrations and association with the social and political realities of the time of their issue, they are often prized for their beauty and historical significance by stamp collectors whose study of their history and of mailing systems is called philately. Because collectors often buy stamps from an issuing agency with no intention to use them for postage, the revenues from such purchases and payments of postage can make them a source of net profit to that agency. On 1 May 1840, the Penny Black, the first adhesive postage stamp, was issued in the United Kingdom. Within three years postage stamps were introduced in Switzerland and Brazil, a little later in the United States, and by 1860, they were in 90 countries around the world. The first postage stamps did not need to show the issuing country, so no country name was included on them. Thus the UK remains the only country in the world to omit its name on postage stamps; the monarch's image signifies the UK as the country of origin.