Structure
There are six pips (short beeps) in total, which occur on the 5 seconds leading up to the hour and on the hour itself. Each pip is a 1 kHz tone (about half way between musical B5 and C6), which, for the first five, last a tenth of a second, while the final pip lasts half a second. The actual moment when the hour changes – the "on-time marker" – is at the very beginning of the last pip.
When a leap second occurs (exactly one second before midnight), it is indicated by a seventh pip. In this case the first pip occurs at 23:59:55 (as usual) and there is a sixth short pip at 23:59:60 (the leap second) followed by the long pip at 00:00:00. The leap second is also the explanation for the final pip being longer than the others. This is so that it is always clear which pip is on the hour, especially where there is an extra pip that some people might not be expecting. Before leap seconds were conceived the final pip was the same length as the others. "Negative" leap seconds can also be used to make the year shorter, but in practice this has never happened.
Although normally broadcast only on the hour by BBC domestic radio, BBC World Service use the signal at other times as well. The signal is generated at each quarter hour and has on occasion been broadcast in error.
Read more about this topic: Greenwich Time Signal
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