4″ x 6″, 5″ x 7″, 8″ x 10″, 11″ x 14″, and 16″ x 20″ picture frames were made to protect contact prints made from negatives of the same sizes.  Contact prints were made with the photo-paper in contact with the negatives, eliminating the need for an enlarger.  The aspect ratio of these frames do not match the aspect ratio of most DSLRs and Point-and-Shoot cameras of today, except for the 4″ x 6″ frame which does match that of most DSLRs today.  The mismatch is why pictures must be cropped, or blank space left in the print, to use the standard picture frames with most modern cameras.