after that, there are between one and three letters for the city or region where the automobile is registered, such as b for berlin. these units usually coincide with the german districts, in few cases an urban district and the surrounding district share the same letter code. usually if an urban district and a rural district share the code, the number of the following letters is different. for example, the urban district (straubing) sr has one letter after the code (sr - a 123). the surrounding district straubing-bogen has two letters (sr - ab 123) after the code. it depends on the number of registered automobiles (or citizens) whether the city or the district has two letters, because there are more possibilities with two letters, so the part with more citizens usually has two letters. for example, the urban district regensburg has more citizens than the rural district regensburg, so the city has two letters after the code r.