Billion and beyond
There has always been confusion between what the British and the Americans call a billion. In the UK, a billion is 1,000,000,000,000 (10^12) but in the US it is only 1,000,000,000 (10^9).
The table below shows the differences between the two sets of numbers; the only one which is common to both systems is the million. However, because of the confusion, the UK now tends to use the US meaning, even though in legal circles the British system continues to be applied.
The table shows the pattern of why there is a difference. In the UK system, an -illion is considered to be 1,000,000 so that each time you increase the value with 6 zeros, you go up the scale. So, a billion is really two -illions, or with 2*6 zeros, a trillion is with 3*6 zeros, a decillion is 10*6 zeros and so on, up to a centillion, which is 100-illions, or 600 zeros.
In the US, the name represents the number of groups of three zeros after the initial 1,000 - so that a billion is two groups of three after the 1,000, or 1,000,000,000 - a trillion is three groups of three, and a decillion is ten groups of three zeros after the 1,000 (a total of 33 zeros). The US centillion is therefore 1,000 followed by 300 zeros (or a total of 303).
The US system was modelled on the old French system, but in the 1940s, the French system was brought in line with the UK system.