The impedance of an electrical system is the nominal voltage to current ratio and is expressed in ohms. Impedance should be thought of as a more general idea than just resistance and can best be understood by making an analogy to the plumbing concepts of pressure (corresponding to voltage) and flow (corresponding to current). A high pressure, low flow system is high impedance whereas a low pressure, high flow system is low impedance. Real circuits may be characterized as being "high" or "low" impedance where high and low are relative, i.e., one part of a circuit could be high impedance relative to another or one circuit could be high impedance relative to another circuit. However, an absolute reference for high and low impedance exists, namely transmission lines. Transmission lines are a conductor pair where distributed capacitance and inductance are important. Because of fundamental physics constants, the characteristic impedance of practical transmission lines is in the range of 25 to 300 ohms. Using this range as a reference, a high impedance circuit can be considered as having characteristic impedance much larger than this range whereas a low impedance circuit can be considered as being in this range or lower.