Carbocations are species bearing a formal + charge on carbon. The have sp2 hybridization and trigonal planar geometry, with an empty p orbital on carbon, perpendicular to the plane containing the substituents (see diagrams shown to the right).
Aortic cross-clamp. An aortic cross-clamp is a surgical instrument used in cardiac surgery to clamp the aorta and separate the systemic circulation from the outflow of the heart. An aortic cross-clamping procedure serves, for example, in the repairing of coarctation of the aorta.
Cross-transfer, or transplantation, is a surgical procedure in which, for example, surgeons amputate a healthy hand from a damaged arm and reattach it to a healthy arm with a damaged hand. While the palm of the hand would face the proper direction, the thumb would be where the pinkie was, and vice-versa.