Why are local primary elections pretty much a slam-dunk for incumbent Democrats?

Clapboardtree Street was originally the "Road to the Clapboard Trees." Trees around Pugatory Brook (near Milk St) were particularly suited to the task of shaving off "claveboards" for enclosing small houses to keep them warm in winter. Early (1600s) Dedham residents such as Ezekialll Holliman cut down their neighbors' trees and ran afould of the community, so once the trees in Purgatory Brook were located, a road was cut to this area and the trees were harvested to keep Dedham residents warm in the winter.