What is the difference between a horizontal sewing machine and a vertical sewing machine?

Answer 1

It relates to the orientation of the hook (the part under the bed that catches the top thread to create the lock stitch) to the needle. Horizontal machines often have a 'fixed' bobbin case that the user doesn't remove, and utilize a drop-in type of bobbin, whereas vertical machines have removable bobbin cases that the user must insert the bobbin into, then insert the filled bobbin case into the machine.

Horizontal bobbins usually hold a lot more thread and make it easier to replace the bobbin without having to remove the work, therefore you will commonly find this type in industrial applications.

Answer 2

Machines come with either a "horizontal bobbin" or a "vertical bobbin."

The horizontal bobbin is normally a "drop in" bobbin -- when you slide the needle plate over, you'll see a circular depression in which you "drop" the bobbin. There's normally an angled slot you'll pull the bobbin thread through.

A vertical bobbin requires that you use a bobbin case. You insert the threaded bobbin in the case, pull the thread through an angled slot in the case, then press the bobbin case into a slot under the needle plate. There's usually some sort of door covering this slot -- although older machines like Singer 301s and Featherweights have the bobbin case simply sitting underneath the bed.