As the largest Canadian cities grew in the 1950s and 1960s, the volume of mail passing through the country's postal system also grew, to billions of items by the 1950s and tens of billions of items by the mid-1960s. Consequently, it became progressively more difficult for employees who handsorted mail to memorize and keep track of all the individual letter-carrier routes within each city. New technology that allowed mail to be delivered faster also contributed to the pressure for these employees to properly sort the mail.