The S-meter (signal strength meter) is an indicator often provided on communications receivers, such as amateur radio receivers, shortwave receivers, and the like. The signal strength (S) scale has evolved from a qualitative system of reporting signal strength from S1 to S9 as part of the RST code system. An informal quantitative guideline exists defining S9 to be equivalent to an input level of 50 microvolts at the receiver's antenna input, typically at 50 ohms impedance. According to this guideline, each S-unit corresponds to a power ratio of 6 decibels (dB), approximately a power ratio of four or a voltage ratio of two. Thus, a signal strength of S8 corresponds to approximately 25 microvolts. Signals stronger than S9 are given in dB, thus "S9 +20 dB".