Whipstock 4 : A long wedge and channel-shaped piece of steel with a collar at its top through which the subs and drill stem can pass, the face of the whipstock sets an angle to deflect the bit. (Bul D20)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}