Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 40A, § 4

Any zoning ordinance or by-law which divides cities and towns into districts shall be uniform within the district for each class or kind of structures or uses permitted.

Districts shall be shown on a zoning map in a manner sufficient for identification. Such maps shall be part of zoning ordinances or by-laws. Assessors' or property plans may be used as the basis for zoning maps. If more than four sheets or plates are used for a zoning map, an index map showing districts in outline shall be part of the zoning map and of the zoning ordinance or by-law.