Let's take a closer look at this photo, shall we?
Credit: Christine Olsson/TT/Sipa

Sharp-eyed royal watchers will notice a key difference between Princess Sofia‘s appearance in last year’s amazing royal family photo for Sweden’s National Day and this year’s photo.

It’s all about the hat! (Actually it’s a scarf, but more on that in a minute.)

Although it has a distinct Flying Nun vibe, the hat is part of a traditional Swedish costume known as a Sverigedräkten.

Traditionally, Swedish women who were not virgins when they married were not allowed to wear the bridal crown, the inspiration for the headwear modeled by the Queen and all the adult Swedish princesses on Monday.

Today, “the headscarf is only worn by married women,” a Swedish Royal Palace spokeswoman tells PEOPLE. “Unmarried women have no scarf. The headscarf is sometimes referred to as a head cloth, but more usually as a scarf.”

Last year’s photo was taken just a week before Sofia’s lavish wedding to Prince Carl Philip on June 13.

Until the 1870s in Sweden, most non-royals’ clothes were handmade to last for life. Every province had its own clothing style, and every village displayed its history and culture through individual outfits.

Clothes were code: When people from Princess Sofia’s native Värmland visited Stockholm, it was easy to identify them through their attire, which to the trained eye showed precisely where they were from, and their financial and social status there.

There were rules about who could wear what and when, and these rules were strictly enforced: Think Fashion Police meets Game Of Thrones.

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But old-fashioned edicts were overturned as Sweden modernized, and by the early 20th century the country’s rulers accepted the need for a national costume that could be donned by all Swedes, wherever they were from.

Consequently, Sverigedräkten is worn in public by the royal family every year on June 6, Sweden’s national day.

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The brightly coloured Sverigedräkten retains some traditional elements of Folkdräkt (a medieval folk costume), which garbed rural women in the countryside as recently as the 1920s.

The main garments are a shift (or blouse) with a waist skirt, or a bodice and skirt in one piece, or a bodice and a jacket.

Traditionally, in some areas, two skirts were an absolute minimum, so a woman could use the top one to cover her head when it started raining.

And in the past, when temperatures have plummeted, Crown Princess Victoria has worn a concealed thermal layer beneath her Sverigedräkten outfit.