Feed dogs are the critical component of a "drop feed" sewing machine

Sewing machine

A sewing machine is a textile machine used to stitch fabric, cards and other material together with thread. Sewing machines were invented during the first Industrial Revolution to decrease the amount of manual sewing work performed in clothing companies...

. A set of feed dogs typically resembles two or three short, thin metal bars, crosscut with diagonal teeth, which move back and forth in slots in a sewing machine's needle plate. Their purpose is to pull ("feed") the fabric through the machine, in discrete steps, in-between stitches.

This arrangement is called "drop feed" in reference to the way the dogs drop below the needle plate when returning for the next stroke. Allen B. Wilson

Allen B. Wilson

Allen Benjamin Wilson was an American inventor famous for designing, building and patenting some of the first successful sewing machines. He invented both the vibrating and the rotating shuttle designs which, in turns, dominated all home lockstitch sewing machines...

 invented it during the time period 1850 to 1854, while also developing the rotary hook

Rotary hook

The rotary hook is a bobbin driver design used in lockstitch sewing machines of the 19th and 20th century and beyond. It triumphed over competing designs because it can run at higher speeds with less vibration.-Operation:...

. Wilson called it a "four-motion feed", in reference to the four movements the dogs perform during one full stitch: up into the fabric, back to pull the fabric along to the next stitch, down out of the fabric and below the needle plate, and then forward to return to the starting position.

Virtually all drop-feed sewing machines can vary their stitch length; this is typically controlled by a lever or dial on the front of the machine. They are usually also capable of pulling the fabric backwards, to form a backstitch

Backstitch

Backstitch or back stitch and its variants stem stitch, outline stitch and split stitch are a class of embroidery and sewing stitches in which individual stitches are made backward to the general direction of sewing...

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