Two researchers cast doubt on our long-held assumptions about Gutenberg's printing methods in a documentary screened on BBC2 this week (19 November).

Using digital photography and computer software, Blaise Aguera y Arcas and Paul Needham analysed Gutenberg's books in an attempt to identify the tools used to create them.

They superimposed the same letters to see if they were identical and therefore made with the same punch.

They found each letter in its many reproductions varied on each page, though there were identical repetitions of that letter on other pages.

They concluded Gutenberg could not have produced his work using a punch and a reusable master matrix as has been widely believed.

Aguera y Arcas claimed the variations in the letters could not be explained by different amounts of ink on the type. But some experts suggest that the irregularities could be simply due to the age of the books.

Gutenberg produced his most famous piece of work, the so-called 42-line Bible, in around 1455.

This is not the first time his methods have been questioned.

In the 18th Century a type founder and printer named Fournier Le Jeune claimed the character irregularities meant individually carved wooden type was used, but this theory has since been disproved.