Original: rating low probabilities by the number of ` nines ' in the decimal expansion of the probability of their not happening : for example , a system which will fail with a probability of 10 5 is 99.999 % reliable : '' five nines '' .
Simplified: rating low probabilities by the number of nines in the decimal expansion of the probability of their not happening : for example , a system which will fail with a probability of 10-5 is 99.999 % reliable : '' five nines '' .