What's the least popular emoji on Twitter?

Emojis often express emotions without words. You may have used a heart, thumbs up or sad face to show how you feel.

But others are more obscure - so you probably haven't used "aerial tramway" or "non-potable water symbol" to express your innermost thoughts.

A tool which tracks emoji use found those two have at various points been the least used on Twitter.

But recently a new candidate emerged for the world's loneliest emoji: "Input symbol for Latin capital letters".

Matthew Rothenberg's Emoji Tracker, external processes every single tweet posted on the social network, in any language, and counts how often each emoji is used.

The tracker has logged 23 billion tweets since he created it in July 2013. The most popular emoji, "face with tears of joy" (you may know it better as the "laughing/crying" emoji), has been posted over 2 billion times since then.

"I was mostly curious to see how people used different emoji," Rothenberg told the BBC.

"Seeing the varied cultural usage of different emoji sometimes can often reveal interesting and unexpected things."

Source: Emoji Tracker

Nine of the ten most popular are hearts and faces. But one is perhaps more unexpected - last year Rothenberg published an article called "Why the Recycling Symbol is Taking Over Twitter.", external

His research found this symbol is frequently used by apps and websites which automatically post Islamic prayers to Twitter, such as du3a.org, external.

The symbol seems to be mostly encouraging people to share posts, rather than having anything to do with recycling.

"It's yet another example of how much variety there is in cultural use of technologies, as well a reminder to me how much may seem a mystery at first due to language barriers," says Rothenberg.

Of course, in addition to the most popular emojis, the website also shows which ones are used least often. That question got wider attention when a Twitter account, "Least Used Emoji Bot", external, was set up to publicise which emoji was the least popular on a given day.

For a long time the "aerial tramway" - a cable car symbol - held that dubious honour.

But then an organised campaign began to dislodge "aerial tramway" from the bottom of the charts by posting the emoji repeatedly on Twitter. High profile accounts started joining in.

Finally, by 22 July, enough people had posted "aerial tramway" enough times to lift it off the bottom of the charts.

The new loser of Twitter's emoji popularity contest is the obscure "input symbol for Latin capital letters". The website Emojipedia, external explains that the symbol is "intended to be used on a software keyboard, or other input screen to toggle capital letter input."

"Some emoji just don't ever find their place in everyday usage and maybe that's ok," says Jeremy Schmidt, who set up the Least Used Emoji Bot Twitter account. "When was the last time you used 'input symbol for Latin capital letters' in a sentence?"

Source: Emoji Tracker

This data only tracks emoji use on Twitter - not Facebook, WhatsApp, iMessage, text messages, or the huge range of other apps where they can be used. Only Twitter has the sort of public interface which allows emoji use to be easily monitored, according to Rothenberg.

Facebook recently revealed the most and least used emojis, external from its internal data. Many of the most popular were the same as on Twitter, like the "crying laughing" and the heart. The company also shared some data on some of the least popular emojis on its network.

"The point of Least Used Emoji was never to disparage down or even really to elevate the least used," says Jeremy Schmit. "In the end, there will always be a least used, and I just think people should know what it is."

Blog by Joey D'Urso

You can follow BBC Trending on Twitter @BBCtrending, external, and find us on Facebook, external.

And you can listen to Trending on the BBC World Service.

All our stories are at bbc.com/trending.