North, Middle, and South Colleges of Old Mill were joined in 1846 to form what was said to be the state's largest building. Parapeted fire walls replaced the open fire breaks, although it was still necessary to leave the building to walk from one section to another.
Some thought the resulting long, narrow, brick building resembled a grist and textile mill, so UVM's main campus building came to be known as "the Mill," and eventually, "Old Mill."