When it comes to disputes regarding who invented which sandwich, one of the fiercest beefs is fought over a Reuben sandwich.

According to one of several theories, this melty sandwich was invented in 1914 at Manhattan's Reuben Delicatessen by its owner, Arnold Reuben. At the request of a famed Broadway actress who wanted to try something new, Arnold made her a savory, sloppy, and highly satisfying sandwich. The actress called it the best sandwich she ever ate and suggested it should be named after her. To which he replied, “Like hell I will, I’ll call it a Reuben’s Special.”

There are several more claims on the Reuben Sandwich and these controversies surely help its popularity. Reuben is a staple of New Yorker cuisine and can be found in restaurants in all of the major cities across USA.

When you hear the word “torta”, you might think of a cake, flatbread, and even a kind of omelet. However, if we talk about Mexican tortas we think of luscious traditional sandwiches filled with delicious, mostly authentic Mexican ingredients.

And when it comes to the origin of this Mexican sandwich, it is believed that French bakers introduced their tradition of baking bread to Mexicans during the French colonization. Mexicans modified it and created their popular varieties, which later became a staple of Mexican cuisine.

One of the staples of French cuisine, Croque Monsieur, first appeared on Parisian menus in 1910, while its first mention in print appears in Proust’s 1918 work In Search of Lost Time.

Some claim it was invented when French workers left their lunch near a hot radiator, only to come back later and discover that the cheese in their sandwiches had melted. Just another great thing discovered by accident!

It probably won't come as a surprise that grilled cheese is No. 1 on the list of the most popular sandwiches in the world since it is so delicious and so easy to prepare.

Although cheese and bread were a common pairing since the early 1900s, and the French have their own, similar sandwich named Croque Monsieur, grilled cheese sandwich first appeared in the United States in the 1920s when it was prepared open-faced as a slice of bread topped with shredded cheese. In the 1960s, the second slice of bread was added on top, and the classic grilled cheese as we know it today was invented!

Named after the initials of its primary ingredients, bacon, lettuce, and tomato, the origins of BLT are hard to pin down. Still, the most common theory is that it was developed from bacon sandwiches originally prepared for teatime in the English countryside in Victorian times.

BLT’s popularity boomed in America after World War II due to two crucial factors: lettuce and tomato became available in supermarkets, and women, who were usually housewives, began to work outside their homes in much greater numbers than before the war.

Bocadillos or bocatas are Spanish sandwiches made with Spanish-style baguettes known as barra de pan, unlike regular sandwiches which are made with modern white bread known as pan de molde in Spain.

Keep in mind that each region in Spain has its own cuisine. That’s why the fillings vary from one region to another, and what is popular in one region of Spain may not even appear in your bocadillo in another region. Who has the original bocadillo recipe? As always, it depends on where you are asking!

In the Midwest and California, known as grinder, in New York and Northern New Jersey, it's hero. In Delaware, it's a sub, and in Baltimore, South Jersey, and Philadelphia, it's called a hoagie. Around New Orleans, it's known as po'boy, and in Maine, Italian sandwich, even if it has nothing to do with Italy.

However, this sandwich with many names originated in the Italian-American communities in the United States in the late 19th century. Why was it at first called a sub? You can probably guess - because it looked like a submarine!

There is no doubt about the origin of the Philadelphian icon known as Philly cheesesteak because the only beef that comes with this sandwich is one inside of it. There is a common agreement that cheesesteak was invented in the 1930s in what is now a Philadelphian institution called Pat's King of Steaks.

Pat's started as a regular hot dog stand in South Philadelphia and one day the owner decided to make a sandwich consisting of thinly sliced rib-eye steak and cooked onions placed into an Italian roll. When a hungry cab driver stopped by and smelled the beef, he forgot about the hot dogs, and a Philly steak sandwich was born.

The “little French sandwich” was born when a returned emigrant from France tried to adapt the croque-monsieur to the Portuguese taste.

However, the “little French sandwich” is not so small at all. On the contrary, it is a unique wholesome meal that can not be enjoyed on the go or as a quick snack. Francesinha is usually found on the menus of traditional restaurants where it is served as a nutritious lunch or dinner.

Bánh mì is a popular Vietnamese variety of sandwiches that share the same core ingredient - a baguette. Yes, the French again!

The baguette was brought over to Vietnam during the colonial period, and the Vietnamese style of sandwich developed in Saigon, also known as bánh mì Sài Gòn, became a popular street food. After the Vietnam War, the sandwich was popularized in Australia, Canada, and the United States.

Although the dish is quite simple and straightforward, tracking down the origin of avocado toast is certainly not. Some claim that the most popular sandwich on Instagram with over 100.000 #avocadotoast posts is an Australian invention, while others proclaim that it was born in the USA.

The truth is, in areas where avocados are commonly found, like Mexico, California, and Australia, people have always eaten the fruit on corn tortillas or toast. However, the mashed avocado on toast that we enjoy today was first suggested by British chef Nigel Slater in the column for Guardian in 1998.

If you ever ordered a club sandwich and wondered how it got its name - you are not the one. The most popular theory about the origin of a true American icon, the club sandwich, claims that it was invented in 1894 at exclusive gambling club Saratoga Club-House in New York.

Another theory says that, although the club sandwich most likely originated at that place, it was not named after the Club. In fact, just like in BLT sandwich, the letters of the 'club' actually reveal what's in it. Therefore, Club stands for "chicken and lettuce under bacon."

Even though today this griddled Italian flatbread is typically enjoyed as a sandwich, piadina Romagnola was once merely a staple of the poor.

In his poem entitled La Piada, which is sort of an ode to the beloved Romagnola piadina, a 19th-century Italian poet Giovanni Pascoli calls it "the bread of poverty, humanity, and freedom," describing it as "smooth as a leaf and as big as the moon."

If you ever visited Cuba and wondered why Cuban sandwich or Cubano is not sold at every street corner, that might be because its origin is disputable. Maybe the historian Loy Glenn Westfall described it the best when he said that the sandwich was "born in Cuba and educated in Key West."

Although first traces of the sandwich can be tracked back 500 years ago and accredited to the original islanders, the Cuban sandwich as we know it today first appeared in the early 1900s, when many Cubans immigrated to Florida. However, it is almost impossible to determine whether Cubano was first made in cigar factories in Florida or sugar mills in Cuba.

Entirely another, but maybe even hotter question is whether the addition of the Italian salami no longer makes it a true Cuban sandwich?

For the full list of world’s most popular sandwiches check: www.tasteatlas.com/sandwiches

The modern roast beef sandwich is thought to have been born in Revere at Kelly’s in 1951, thanks to the canceled wedding and whole roast beef leftovers. The roast beef was sliced thin and served up plain on bread. It was an instant hit.

When it comes to toppings - everything goes. Still, in Boston, where roast beef sandwiches are one of the area's specialties, they are traditionally topped with cheese, barbecue sauce, and mayonnaise.