For example, when testing semiconductor memory: voltages, temperature, and refresh rates can be varied over specified ranges and only certain combinations of these factors will allow the device to operate. Plotted on independent axes (voltage, temperature, refresh rates), the range of working values will enclose a three-dimensional, usually oddly-shaped volume. Other examples of conditions and inputs that can be varied include frequency, temperature, timing parameters, system- or component-specific variables, and even varying knobs tweakable during silicon chip fabrication producing parts of varying quality which are then used in the process.