At the heart of almost every government decision about battling the coronavirus is a simple number, which just a few weeks ago was a relatively obscure calculation used by people who study diseases.
That is the R number or R0 (pronounced R-nought), which measures a diseases ability to spread. When R is 1 every infected person infects one other person - spreading the coronavirus.
If R is below 1 then every infected person is passing it on to less than one person and the spread of the infection slowly but surely sinks.
For weeks Downing Street and the Government's economic advisers have been clear - R must drop below one and stay there for the lockdown to be eased.
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