Demersal trawling is a direct descendant of the early beam trawl. The original form of towed fishing gear used by sailing boats with their unpredictable towing power was generally a beam trawl.
We have a comprehensive learning package on fishing skills including fishing gears available here: Introduction to Fishing Methods
The size of the gear being restricted to the length of beam that could be stowed aboard the vessel, and the limitations on available wind power. With the introduction of steam powered vessels, and later diesel propulsion, otter boards were developed to spread the nets in place of the rigid beam of the beam trawl.
This allowed nets to be made much larger. To begin with, the otter boards or trawl doors, as they are more commonly called nowadays, were attached to the wing ends of the nets.
Later, with the understanding that certain species of fish could be herded into the path of the net by ropes or wires on the seabed, short bridles or sweeps were added between the wing end of the net and the trawl doors, allowing a larger area of seabed to be swept by the gear.
Nowadays, the sweeps and bridles can range from none at all, right up to 300 metres, depending upon the target species and the type of seabed being fished.