Before the fifteenth century, books in Europe were copied by hand, which took months and made them very expensive. Around 1450, Johannes Gutenberg, a German craftsman, built a printing press that used movable metal letters. The letters could be arranged to make a page, covered with ink and pressed onto paper. Afterwards, they could be rearranged for the next page. Printing many copies became much faster and cheaper. As books became cheaper, more people learned to read, and new ideas spread quickly across countries. Many historians regard the invention as one of the most important in history. Printing with blocks and movable type had been used earlier in East Asia, but Gutenberg's press had a large effect on Europe and, later, on the rest of the world.

Question: In the sentence 'Afterwards, they could be rearranged', what does 'they' refer to?