In 1955, AT&T distributed a list of recommended exchange names that were the result of studies to minimize misunderstandings when spoken. The recommendation was intended for newly established exchanges, and did not mandate any renaming of existing historical names. The number sequences 55"x", 57"x", 95"x", and 97"x" had no exchange names specified, as the mappings for the digits 5, 7, and 9 had no vowels, thus making it difficult to find names with those consonant combinations. As a result, those numbers were very seldom assigned to exchanges; however, KLondike was used for 55x in San Francisco and Columbus, Ohio, and WRigley 5 (975) in Chicago (Wrigley Field). On the telephone dial, letters were mapped to digits using the assignments shown in the table (right).