Imperforate Penny Blacks

Penny Black stamps were imperforate. This meant that the postmaster or mistress had to cut out the stamps from the sheet by hand. As a result of careless cutting, many of the stamps have incomplete margins, the cuts often extending into the printed area of the stamp.

Because the Penny Blacks were not perforated, the seller had to cut out the stamps from the sheet using scissors or a knife. There was only about 1mm between the stamps and it was not unusual to accidentally cut into the printed section of the stamp, resulting in irregular margins. The number, size, and regularity of the margins make all contribute to the value of the Penny Black stamp. For example, a Penny Black with two full margins and a reasonable amount of the other two margins is regarded as an average specimen. Stamps with four, regular margins are exceptional, and collectors will pay higher prices for these stamps.

Between 1848 and 1854 trials were carried out in order to find a better and more acceptable way of separating stamps than the scissors and in early 1854 the first ‘perforated’ penny red was issued. 

Source: http://www.pennyblackstamp.co.uk/printing_penny_black_stamps.html