The biggest issue with a single bin worm farm design is that it is quite hard to harvest. A horizontal worm bin addresses this by dividing the worm farm into multiple compartments.
The simplest horizontal worm bin consists of a bin which has been divided into two areas by a perforated partition.
When the farm is started worms and food are added on the one compartment of the worm bin. This side is used until it is full. Once full, the other compartment is started. The worms will migrate through the perforated partition to where the food is. After a period of time most of the worms will be in the second compartment at which point the first compartment is harvested.
Once the second compartment is full food is introduced into the first compartment again and the worms will move back to the first compartment.
Windrows are very often farmed in this manner. Once a windrow is mature a second windrow is built parallel to and touching it. The worms migrate to the new Windrow and the original one can be harvested.