Collar 03 : A coupling device used to join two lengths of pipe. A combination collar is a coupling with left-hand threads in one end and right-hand threads in the other. Sometimes drill collars are called simply collars. (WLOP)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}