As steam and diesel progressively replaced the tow-horse in the early years of the 20th century, it became possible to move even more cargo with fewer hands by towing a second, un-powered boat, referred to as a "butty", "buttyboat" or "butty boat". Although there was no longer a horse to maintain, the butty had to be steered while being towed. So that the butty boatman could lengthen or shorten towline as needed, the towline wasn't tied-off on the bow, instead travelled over the buttyboat through permanent running blocks on stands or retractable middle masts and managed in the stern. On a wide canal, such as the Grand Union Canal, the pair could be roped side-to-side ("breasted up") and handled as a unit through working locks.