A large family of viruses that infect bacteria, called phages, is characterized by long tails used to inject DNA into their victims cells. The tape measure protein got its name because the length of the corresponding gene is proportional to the length of the phages tail: a fact shown by actually copying or splicing out parts of DNA in exemplar species. A natural question is whether there exist units for these tape measures, and if different tape measures have different units and lengths. Such units would allow us to retrace the evolution of tape measure proteins using their duplication/loss history. The vast number of sequenced phages genomes allows us to attack this problem with a comparative genomics approach.