Grovers Double Locking Chainstitch

A simple chain stitch will run. That is, it will come undone if one part of the stitching is broken. A lockstitch will not run but it uses a shuttle which has to be small to pass through the loop in the top thread. This means that only a small amount of thread can be used and this will need refilling frequently. Grover's method uses two interlocking chainstitches. The end of the lower thread does not go through a loop so it can be fed from a large bobbin.

The Parts
The needle passes the top thread through the material in the usual way. An eyed looper carrying the lower thread pushes a loop through this first loop from the top thread but does not withdraw. The needle then forms another loop through the first loop from the lower thread, locking it so that the looper can withdraw. The needle having made its second loop remains whilst the looper pushes another loop through it to lock it. The needle can then withdraw to make another loop further along the cloth.
The Double Locking Chainstitch
The animation shows the needle and looper moving right whereas in reality it is the cloth which moves carrying the completed stitching to the left. Grover used a spiral looper. Modern machines often use an oscillating arm.
Grover's Spiral Looper