of stereo mic pairs, or the azimuth of analogue tape machine heads, which rely on cancellation of the S signal. M3 and M6 M and S meters are normally aligned to the 'M6' standard in which M = (L + R) − 6 dB and S = (L − R) − 6 dB. In other words, the sum and difference signals are each attenuated by 6 dB before being displayed on the meter. As a result, signals of identical amplitude and phase in the left and right channels make the M meter show exactly the same deflection as for the individual L and R meters. This is because summing two identical signals produces a result 6 dB louder than either source, but the M and S meters show summed signals attenuated by 6 dB to compensate. The M6 standard means that dual mono sources (e.g. a presenter panned to the centre of a stereo sound stage) can be peaked to 6 in both channels, with the M meter also showing 6. The M6 format has largely replaced the earlier 'M3' standard in which the sum and difference attenuation is only 3 dB. This M3 format is designed to give a more accurate indication of the level of the summed mono signal when working with conventional stereo material. The premise is that in summing two signals of similar level but carrying non-phase-coherent sounds (i.e. typical stereo material), the result averages 3 dB more than either source channel (rather than 6 dB more). The M3 standard means that true stereo material can be peaked to 6 in both channels, with the M meter also showing 6. However, dual-mono sources can only be peaked to 5.25 in each channel to keep the M meter at 6. Note: the chosen M6/M3 metering standard does not affect the relative audible balance of sounds panned to one side versus the centre – that is determined solely by the panning law of the mixing console's pan-pot. The M6 standard is deemed a simpler form of metering for untrained broadcasters to use as it keeps the M meter at '4' for Alignment Level and '6' for peaks, without the operator having to remember to subtract 3 dB. Commercial broadcasting in the UK initially used M3 but had switched to M6 by 1980. This was mandated by the IBA's Engineering Code of Practice. BBC installations used M3 until 1999.