Window cutting system for downhole tubulars

A system for cutting a window in a downhole tubular using at least one mill 12 having a guide 40 with a peripheral surface thereon, comprises a whipstock 20, and a lug L on said whipstock having a tapered contact surface (36, Fig 3). The lug further comprises a back-up shoulder (38, Fig 3) which forms a guide surface for the peripheral surface of the guide on the mill. The lug may be generally L-shaped in cross section and comprise an arcuate surface to contact the guide on the mill. The lug may be worn away as the guide of the mill advances, to force the mill toward the tubular, due to the tapered surface. The system may comprise a retrieval device, which remains functional throughout the milling process.