triple-decker
What does triple-decker mean?
A three-story wooden apartment house, one unit per floor, built by the thousands across Boston, Worcester, Providence and the rest of New England between roughly 1880 and 1930 to house mill and factory workers. Long porches stacked out back, flat or low-pitched roof, and almost always wood-frame. If you grew up in Dorchester or Somerville you grew up in one or next to one.
Origin
New England working-class housing boom of the late 19th century; the form became so dominant in cities like Worcester and Boston that it's now a recognised vernacular architectural style.