Folders, also called souvenir folders, feature several postcard-sized views printed on thinner paper and folded accordion-style, in a thicker paper cover. In this way, several pictures could be sent for less than the cost of mailing three postcards.
This College of Saint Benedict folder was published in the late 1920s or the 1930s. It features twelve views.
The pictures are halftone reproductions.
This St. John's University folder from about 1947 could be mailed for 1 ½ cents, the advertising rate, if no message was included. This was cheaper than sending a single postcard.
The views were printed as duotones: two colors of ink give a longer range of tones. The Albertype Company printed postcards using collotype. Collotype involves a printing plate with reticulated light-sensitive gelatin. Done well, it can be nearly indistinguishable from a photographic print directly from film. You can see evidence of retouching in this set, especially along rooflines.
For more on folders, see:
Toal, Bob, "Nobody Collects View Folders," Postcard History, August 1, 2019.