The chain martingale is also known as a training collar or half check collar.  It has a loop of chain attached to the webbing body of the collar that will tighten when the o-ring in the middle is pulled.

An accurate measurement of both the neck and the widest circumference of the head (generally just before the ears on the broadest part of the skull) are required for sizing.  Correctly sized martingales should have at least 2 fingers widths between the O-rings attached to the webbing so that when pulled taught they never touch.  This collar comes with a D-ring for tag attachment.

Please use care with this tool and remember that your best friend is on the other end.  Dog necks are sensitive and harsh corrections may injure them.