When Prince Charles and Princess Diana were married in 1981, the Princess of Wales had to learn all the ropes of royal life including what she could and could not wear according to protocol. There was one thing though that she couldn’t wear and it had nothing to do with some royal fashion rule.

Here’s what the princess wasn’t able to wear and why.

Princess Diana couldn’t wear these next to Prince Charles

The thing Princess Diana could never wear next to Prince Charles was high heels and the reason for that is because of her height.

The prince and princess’ exact height has been debated for years with some publications listing them at 5 foot 10 and others claiming that they were both 5 foot 8. Either way, being the same height meant that if Diana wore shoes with high heels she would tower over the future king so she had to refrain from wearing them.

This explains why the princess was seen in flats most of the time when she stood right next to Charles.

But if they were the same height than why does the prince appear much taller than his wife in all of their official portraits?

Why Charles looked taller than Diana in some photos

Fans on social media brought up the question after noticing a number of their professional shots together in which the princess appears to be much shorter than Charles. In those photos, Diana only looks to be around the height of her husband’s shoulders.

According to The Society Pages, their photos were edited or the pair was situated in such a way to make Charles look taller. This was done in order to keep with an idea back then that men should have some sort of height advantage over women and be taller than their wives.

“This effort to make Charles appear taller is a social commitment to the idea that men are taller and women shorter,” said Professor Lisa Wade of the Occidental College. “When our own bodies, and our chosen mates, don’t follow this rule, sometimes we’ll go to great lengths to preserve the illusion.”

She added that “When Charles and Diana were posed together formally, however, they were typically arranged so as to suggest that he was significantly taller than her, or at least to disguise the fact that he was not.”

Therefore in official portraits, Diana is sitting down or leaning her head to the side. And in their engagement photo (above), Charles is standing on a step behind Diana. Of course, the Prince and Princess of Wales couldn’t pull off optical illusions when they were out together in public so in those instances, Diana only wore flats or very low pumps to more formal affairs.

Read more: The Real Reason Princess Diana Wore Two Watches at a Time on Her Wrist