The shamrock has been the national symbol of Ireland ever since Saint Patrick used its three leaves to illustrate the Holy Trinity.
The story doesn’t work with four leaves.
So when it emerged that Donald Trump’s new line of St Patrick’s Day hats featured a four-leaf clover, eagle-eyed Irishmen and women were quick to call him out on social media.
The Trump Store’s website offers the green “Make America Great Again” cap for $50.
“Capture the luck of the Irish with this Make America Great Again Hat,” runs the bubbly blurb, before advertising its botanically-challenged logo: “Embroidered Four-Leaf Clover on back.”
@TeamTrump $50? 50 bucks for a baseball cap. A $2 cap for $50. Small wonder Donny is a billionaire. And that's a clover not a shamrock. ☘— Brian Fortune (@BrianFortune1) March 3, 2017
To be fair to Mr Trump (not an easy expression to write), it is a common mistake.
After all the shamrock is actually a young clover – the word means “little clover” in Gaelic - although not all clover species are shamrocks.
As a result the four-leaf version, supposedly a symbol of luck, is used throughout North America on St Patrick’s Day and to represent things of an Irish nature.
In 2012, thousands of Barack O’bama T-shirts had to be reprinted for March 17 after complaints that they showed clover rather than shamrock.
The French luxury goods manufacturer Hermes attracted ridicule in Ireland when it used a four-leaf clover design on a special one-off Irish handbag, made from green crocodile skin, and worth £24,000.
Even Guinness muddled the two in a Canadian St Patrick’s Day promotion in 2014 using a four-leafed design in adverts.