in the United Kingdom since this is twice the UK mains power line frequency; an inadequately smoothed power supply may cause unwanted squelch opening (this is true in many other areas that use 50 Hz power). Tones typically come from one of three series as listed below along with the two character PL code used by Motorola to identify tones. The most common set of supported tones is a set of 39 tones including all tones with Motorola PL codes, except for the tones 8Z, 9Z, and 0Z (zero-Z). The lowest series has adjacent tones that are roughly in the harmonic ratio of 20.05 to 1 (≈1.035265), while the other two series have adjacent tones roughly in the ratio of 100.015 to 1 (≈1.035142). An example technical description can be found in a Philips technical information sheet about their CTCSS products. Notes Non-standard numerical codes. Many radios use a matching set of numerical codes to represent corresponding tones; however, there is no published standard and only partial industry adoption. Some radios use 69.4 Hz instead, which better fits the harmonic sequence, and this tone is often omitted as a choice. Also known by the code SP. Not actually in this harmonic sequence, but an average of the ZA and 1Z tones used to fill the gap between the lower and middle sequences. 98.1 Hz would be the tone after ZA, and the tone before 1Z would be 96.6 Hz, assuming the same harmonics were used. Many NATO military radios have a switchable 150.0 Hz tone. The list includes the following radios: AN/PRC-68, AN/PRC-117F, AN/PRC-117G, AN/PRC-77, AN/PRC-113, AN/PRC-137, AN/PRC-139, AN/PRC-152, AN/PRC-119, AN/VRC-12, AN/PSC-5, and Thales AN/PRC-148 MBITR. The 8Z, 9Z, and 0Z ("zero-Z") tones are often omitted from radios that use the M1–M7 series of tones. Non-standard tone not included in current TIA-603-E. Reverse CTCSS Some professional systems use a phase-reversal of the CTCSS tone at the end of a transmission to eliminate the squelch crash or squelch tail. This is common with General Electric Mobile Radio and Motorola systems. When the user releases the push-to-talk button the CTCSS tone does a phase shift for about 200 milliseconds. In older systems, the tone decoders used mechanical reeds to decode CTCSS tones. When audio at a resonant pitch was fed into the reed, it would resonate, which would turn on the speaker audio. The end-of-transmission phase reversal (called "Reverse Burst" by Motorola