There is always some confusion among most of the Philatelist or Stamp collectors about the difference in Miniature Sheets and Souvenir Sheets, confusion as different philatelic entity has a separate description for Miniature sheet and Souvenir sheet.
When I browsed the net looking for a description and difference on those terms I found that it is defined in many ways and also depends on the country where the term was used.
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In India most of the collectors use the term Miniature Sheet (refer picture above) including India Post, for the small sheet of stamp which may include one or more stamp, usually different stamps. The stamp may be part of the picture depicted in the sheet or a separate one, and the paper margin surrounding the stamps may be plain design. Here the stamp has perforation and can be separated from the sheet and when it is separated it usually it looks same as the separate sheets of stamp issued along with the Miniature sheet. The miniature sheet is issued purely for the collectors. We rarely use the term Souvenir sheet in India.
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The Linns website has a different description:
A miniature sheet is a smaller-than-normal pane of stamps issued in that form in addition to being issued in full panes. The smaller size and fewer stamps are what distinguishes it from a regular pane. (This is usually referred as sheetlets among Indian collectors and what we refer as Miniature sheets are Souvenir Sheets according to the Linns)
A miniature sheet is usually without marginal markings or text saying that the sheet was issued in conjunction with or to commemorate some event, which is what distinguishes it from a souvenir sheet.
In practice, many dealers and collectors refer to both small regular panes and souvenir sheets as "miniature sheets." (Refer www.linns.com)
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India had issued a Miniature sheet (or is it a souvenir sheet?) in 1973 for the India International Philatelic Exhibition, where the 4 stamps depicted in it did not have any perforation and could not be separated. Some collectors/Senior Philatelists define this as Souvenir sheet, as the stamps cannot be separated and this is purely a Souvenir to be collected by the Philatelists.
I would be delighted to get maximum comments on this subject as to what are your views and what it is known as, in your respective countries for those who read this article in different corners of the globe. For now, whatever name it is called by, most collectors love to have this piece (Miniature or Souvenir sheet) among their collections so do keep collecting and enjoy it.