Miniature sheets are a way of presenting a set of stamps by joining together within a frame to form a picture or a striking image. Sometimes the frame gives information about the stamps. They were originally issued to help raise funds for the huge International Stamp and Cover Exhibition, which is hosted by Britain every decade. They proved so popular that miniature sheets are currently issued by Royal Mail around 6-7 times a year and offer a different way of displaying new stamps. They are nearly always se-tenant in format.
[RHS Cover] Miniature sheets are available on covers. Some collectors specialise just in miniature sheet covers, others prefer to leave them and just buy the individual stamp covers, while other collectors like to own both versions for each new issue. These Royal Horticultural Society first day covers are an example of both styles. In the Minature Sheet version (top), you will see all the stamps are joined together within a black frame.
Compare it to the lower illustration, which is the stamps version of the same cover. You will see that all the stamps below are the same as the ones inside the miniature sheet above. The difference is all in the presentation!
This is an early example of postal stationery (Issued 6th May 1840, the same day as the Penny Black). It was available either as an envelope or a foldable letter (like an air letter). These already had the postage paid and have a special illustration on the front. Though very rare, they do exist postmarked on the first day of issue.