In probability, the nine is a logarithmic measure of the probability of an event, defined as the negative of the base-10 logarithm of the probability of the event's complement.  For example, an event that is 99% likely to occur has an unlikelihood of 1% or 0. 01, which amounts to −log10 0. 01 = 2 nines of probability. Zero probability gives zero nines (−log10 1 = 0). A 100% probability is considered to be impossible in most circumstances: that results in infinite improbability. The effectivity of processes and the availability of systems can be expressed (as a rule of thumb, not explicitly) as a series of "nines". For example, "five nines" (99. 999%) availability implies a total downtime of no more than five minutes per year – typically a very high degree of reliability; but never 100%.