Curve tracers are capable of generating lethal voltages and currents and so pose an electrocution hazard for the operator. Modern curve tracers often contain mechanical shields and interlocks that make it more difficult for the operator to come into contact with hazardous voltages or currents. The device under test can also become hot during operation, posing a burn hazard to the operator. Curve tracers usually contain convenient connection arrangements for two- or three-terminal device under test, often in the form of sockets arranged to allow the plugging-in of the various common packages used for transistors and diodes. Most curve tracers also allow the simultaneous connection of two device under test, in this way, two device under test can be matched for optimum performance in circuits such as differential amplifiers which depend upon the close matching of device parameters. This can be seen in the image to the right where a toggle switch allows the rapid switching between the DUT on the left and the DUT on the right as the operator compared the respective curve families of the two devices.