Because the functions are narrowly defined, ground testing instrumentation exists in only a few fundamental designs. The basic tests performed are ground resistance, soil resistivity, and bonding, or the continuity of the grounding conductors connecting equipment to ground. Traditionally designed testers perform two or all three of these tests, depending on whether 3- or 4-terminal models. The latter add soil resistivity to the repertoire; the former do not. Clamp-on testers are a later addition, designed mainly for speed and ease of use. They do not require walking out hundreds of feet with leads and probes in order to connect the terminals to ground. Clamp-ons perform ground resistance and have a limited continuity function (they can indicate if the return circuit is open or high resistance, but have no means to test specific points). They cannot do soil resistivity. Another later addition doesn’t replace either of these technologies but enhances the capabilities of the traditional terminal design. This is the incorporation of a current clamp into a 3- or 4-terminal tester so that test current can be selected (by clamp placement) between various components of a parallel system, and hence the resistance of that component alone can be measured separate from the total resistance of the parallel system of grounds.