The R number has been a headline grabbing figure throughout the UK's Covid-19 outbreak, with repeated focus on keeping it below one.
Scientists facing the pandemic estimate how quickly coronavirus is spreading by calculating the basic reproduction number called the R0, or R nought.
It reflects how infectious a virus is - but what does it actually mean?
What is the reproduction number?
It is defined as the average number of people an infected person can expect to pass the virus on to before any widespread immunity or attempts at immunisation are made.
For example, if one person develops an infection and transmits it to two other people, the number would be R2.
The higher the number - the faster the virus progresses.
If greater than one, the infection will "spread exponentially".
If it is lower than one, it will spread slowly and eventually die out, according to the Centre for Evidence-Based Medicine (CEBM) at Oxford University.
The latest R number for England is between 1.3 and 1.6, meaning that on average every 10 people infected will infect between 13 and 16 other people.
What is the R0 for Covid-19?
The World Health Organization (WHO) estimated at the start of March the R number was between 2 and 2.5, but estimates from different studies vary widely.
According to nine studies in China and South Korea between December and March, the mean estimated R number is 2.63, the CEBM said.
An Imperial College London study widely seen as influencing the government’s lockdown measures predicted around 490,000 deaths in the UK with a model using an R number of 2.4.
By comparison, measles has an estimated value of more than R14 while smallpox is almost R6, the CEBM said.
What is the rate of infection now?
In the UK, the R number has risen as a second wave of Covid infections hits the country.
The R number of coronavirus in England has risen to 1.3-1.6 for the week ending 2 October, according to the latest data from the government.
Every regions across the country now has an R number of 1 or higher, which means there is widespread growth of the virus.
Strict social distancing measures, a dramatic drop in use of public transport, and people leaving the house only for essential trips are all cited as effective measures at slowing transmission.
However as restrictions eased from the country's nationwide lockdown, and as more people come into contact with each other, there are more chances for the virus to spread.
How is the R value calculated?
There are many different mathematical and statistical models used to predict the R value.
One is dubbed the SIR model which accounts for three factors: the number of susceptible individuals, the number of infected people, and the rate of removal from the population, either by recovery or death.
But estimates between studies vary widely owing to the quality and reliability of data and different types of epidemiological models, the CEBM said.
The outcome value is affected by the proportion of susceptible people, the density of populations, the infectiousness of the organism, and the rate of removal of cases either by recovery or death, it added.
Coronavirus: Everything you need to know