As the UK gradually moves out of coronavirus lockdown, one figure is at the forefront of ministers' and advisers' minds.
The R number measures the rate of transmission of coronavirus in the UK - or to put it more exactly, how many other people an average person who has the virus will pass it onto.
Original estimates of coronavirus' R number among a population with no immunity to the virus ranged between 2 and 6.6.
But the R number has dropped dramatically due to lockdown and social distancing measures, with the UK's figure currently thought to be between 0.7 and 1, as of May 22 the latest figure available.
Here we look at the R number, how it's calculated and why getting it down to 1 is so important.
How is the R number calculated?
The R number cannot be calculated in real-time.
Instead, scientists look at figures including positive tests, rates of hospitalisation and fatalities to work out an estimate of the rate of transmission.
Generally this creates a picture that covers roughly the past month. The accurace of estimates should improve over the course as testing and tracing rates rise.
What is the UK's R number?
As outlined above, it's difficult to tell what the UK's exact R number is, given scientists are still unsure about how many cases there are in the UK.
However, researchers at Imperial College in London have created a model of the R number, and estimated that it is stable between 0.7 and 1 in the UK.
This is up slightly from a few weeks ago, with the virus still being transmitted in care homes and hospitals.
Why is it important to keep the R number below 1?
If the R number is below one, it means that on average, someone who has the virus passes it onto fewer than one other person.
On a larger scale, that means that the number of people who are infected with the virus will go down.
Does the R number affect lockdown?
As the lockdown lifts, the Government and its scientific advisors will be keeping an eye on the R rate.
If it rises sharply, that could mean some restrictions are brought back in.
Dr Adam Kucharski, of the London School of Hygiene and Tropical Medicine, told the BBC: "It's a big challenge making sure you're not loosening too much and increasing transmission."
"It doesn't give you a lot of room to play with [to keep the number below one]", Dr Kucharski added.
