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4. Her Above-The-Knee Dress
There's a reason why you barely ever see a royal wear a mini-skirt, and that's because those hems are against fashion etiquette rules. The queen prefers for her family members to wear skirts and dresses that are either to the knee or below, since it's a more conservative length. 

Midis and maxi skirts are classic silhouettes, whereas a shorter hem can become dated. The whole point of the royal family is to be timeless, so wearing something too trendy or youthful can chip away from that dignified air. It also wouldn't do for the royal members to be sexualized by headlines, so erring on the side of caution when it comes to skirt lengths nixes any chance of scandal or criticism. 

That being said, Middleton doesn't always follow the rules. While she has never worn a thigh-skimming mini, she has on occasion worn dresses that hover above the knee.

The demand for a longer hemline could also be because of the time the queen was born. She was born in 1926, so a short hemline was something radical and rebellious when it came onto the scene in the 1960s. Whereas nowadays it's just seen as a normal skirt option, the queen might still hang onto the length's old connotations. Whatever the reason might be, Middleton often decides to play it safe and wears a midi — but every now and then she wears a shorter skirt if the occasion calls for it.