Image copyright What3words Image caption What3words divides the world into 57 trillion squares, each with a unique three-word address

Police have urged everyone to download a smartphone app they say has already saved several lives. What is it and how does it work?

Kicked. Converged. Soccer.

These three randomly chosen words saved Jess Tinsley and her friends after they got lost in a forest on a dark, wet night.

They had planned a five-mile circular stroll through the 4,900 acre (2,000 hectare) woodland Hamsterley Forest, in County Durham, on Sunday evening, but after three hours they were hopelessly lost.

"We were in a field and had no idea where we were," the 24-year-old care worker from Newton Aycliffe said.

"It was absolutely horrendous. I was joking about it and trying to laugh because I knew if I didn't laugh I would cry."

Image copyright Jess Tinsley Image caption Jess Tinsley dialled 999 after getting lost - and was told to download an app to her smartphone

At 22:30 BST they found a spot with phone signal and dialled 999.

"One of the first things the call-handler told us to do was download the what3words app," Ms Tinsley said.

"I had never heard of it."

Within a minute of its download, the police said they knew where the group was and the soaked and freezing walkers were swiftly found by the Teesdale and Weardale Search and Mountain Rescue Team.

Image caption Hamsterley Forest covers a large area of County Durham

"I have told everyone I know to download this app," Ms Tinsley said.

"You never know when you are going to get lost and need it."

What3words essentially points to a very specific location.

Its developers divided the world into 57 trillion squares, each measuring 3m by 3m (10ft by 10ft) and each having a unique, randomly assigned three-word address.

For example, the door of 10 Downing Street is slurs.this.shark, while the area across the road where the press congregate is stage.pushy.nuns.

It was born out of company founder Chris Sheldrick's postal-related problems growing up in rural Hertfordshire.

"Our postcode did not point to our house," he said.

"We got used to getting post meant for other people, or having to stand in the road to flag down delivery drivers."

Image copyright What3words Image caption Chris Sheldrick founded what3words in 2013

Ten years in the music industry, which involved trying to get bands to meet at specific entrances to their venues, also fuelled his frustration.

"I tried to get people to use longitude and latitude but that never caught on," Mr Sheldrick said.

"It got me thinking, how can you compress 16 digits into something much more user friendly?

"I was speaking to a mathematician and we found there were enough combinations of three words for every location in the world."

In fact, 40,000 words was enough.

The company started in 2013 and now employs more than 100 people at its base in Royal Oak, west London.

Mongolia has adopted what3words for its postal service, while Lonely Planet's guide for the country gives three word addresses for its points of interest.

Image copyright Getty Images Image caption Mongolia has adopted what3words for its postal system

Mercedes Benz has also included its system in its cars and what3words is now being used in 35 languages.

But still, not enough people know about it according to Lee Wilkes, a crew manager for Cornwall Fire and Rescue Service, one of 35 English and Welsh emergency services to have signed up to the system.

"It cuts out all ambiguity about where we need to be," he said.

Tackling fires in large rural expanses, for example on moors, will be helped by the system, Mr Wilkes said.

Image copyright Lonely Planet Image caption Lonely Planet uses what3words addresses for Mongolian points of interest

"Instead of saying 'meet at the gate and then get directed from there,' we can be absolutely specific about where our crew needs to get to," Mr Wilkes said.

"It will make for a much more effective service. We are quite excited about it.

"It would be flippant of me to say this will become commonplace but I really do think it could be.

"I just cannot see a downside."

Image copyright Getty Images Image caption People who find themselves needing help in remote areas can use the app to direct emergency services to them, police say

If people do not have the app, the emergency services can send a text message containing a web link to their phones.

But that requires a signal (85% of the country is said to have a 4G connection). The app does not need a phone signal to tell someone their three-word location, however.

"Say there was a group up a mountain and one got injured," Mr Sheldrick said.

"They haven't got any signal to call for help, but they can still find out their three word location.

"Someone from the group can then take that down and tell the emergency services, who then know exactly where to go to find the injured person."

The emergency services are urging people to download the free app.

South Yorkshire Police used it to find a 65-year-old man who became trapped after falling down a railway embankment in Sheffield.

North Yorkshire Fire and Rescue Service found a woman who had crashed her car but was unsure where she was.

And Humberside Police were able to quickly resolve a hostage situation after the victim was able to tell officers exactly where she was being held.

"That was a time critical situation and being able to use a three word address meant officers could get there much quicker, rescue the hostage and arrest a man," Mr Sheldrick said.

"That made us understand how the work we are doing is so important."

Image copyright What3words Image caption The app enables the emergency services to know exactly where they are trying to get to

Humberside Police also used the system to find a group of foreign nationals, including a pregnant woman in labour, who were trapped inside a shipping container at a port.

"The port had over 20,000 containers and we knew that we needed to get to them quickly," said the force's control room supervisor Paul Redshaw.

The group were told to download the app and they were soon found.

"There is no doubt in my mind that these incidents could have had very different outcomes had we not been able to use what3words," Mr Redshaw said.