“The ‘T’: One of Boston’s Last Great Bargains” proclaims this neat little pop-arty pocket map cover. Five of the six colours used in the design make sense (representing the four lines of the “T” plus purple for commuter rail), but what’s the yellow for? Making up the numbers in the days before the Silver Line, it seems.

EDIT: My readers are awesome! Thanks to those who have pointed out that yellow was (and still kind of is) used to represent buses in the MBTA network, which explains its appearance here rather nicely.

Source: Mikey Ashworth/Flickr