Section 1. the City of Boston Shall Be Divided Into Districts of Two

classes, to be designated districts A and B. The boundaries of the said districts, established as hereinafter provided, shall continue for a period of fifteen years, and shall be determined in such manner that those parts of the city in which all or the greater part of the buildings situate therein are at the time of such determination used for business or commercial purposes shall be included in the district or districts designated A, and those parts of the city in which all or the greater part of the buildings situate therein are at the said time used for residential purposes or for other purposes not business or commercial shall be in the district or districts designated B.