The Boston Block, also known as Aalfs Manufacturing Company, is a historic building located in Sioux City, Iowa, United States. The city experienced a building boom that began in the late 1880s and continued into the early 1890s. One of the major players in that building boom was the Boston Investment Company, a company on the East Coast who built four large commercial blocks in Sioux City simultaneously.[2] Construction on the four buildings began in 1890 and they were completed the following year. In addition to the commercial blocks, they also built a steam heating plant that provided steam and light to three of the buildings as well as to neighboring buildings. The Massachusetts Block (no longer extant) on the southwest corner of Fourth and Jackson was six stories tall and had a similar facade as the Boston Block, which is five stories tall on the northeast corner of Fourth and Virginia. The Plymouth Block on the southeast corner of Fourth and Locust was also five stories tall, and the Bay State Block on Fourth Street is the shortest at four stories.[2] Among the building's tenants was the Aalfs Manufacturing Company, which used the building as its headquarters.