There’s one old-fashioned rule in place for guests attending the royal wedding

Weddings are a minefield of social etiquette at the best of times and a royal wedding is no different.

Prince Harry and Meghan Markle will be joined by up to 800 guests at St George's Chapel on their big day on 19 May. The nuptials will likely be a more relaxed affair than Kate Middleton and Prince William's but those 800 guests may still want to brush up on the rules of attending a royal wedding. One thing all ladies will have to do is wear a hat, whether they like it or not. While it's optional for most ordinary ceremonies and often seen as old-fashioned, in a royal wedding it's an absolute must, according to former royal editor Duncan Larcombe. He told Cosmopolitan that the style of hat is also very important - anything too ostentatious or big enough that it would block others' views is a no-no. Something else that guests must abide by is a strict seating plan, he said. "The seats are all allocated. They are numbered to match the number given on the invitation." "The entire royal family will be seated to the right-hand side of Harry and Meghan. Meghan's parents, co-stars and friends will be given priority seating on the left. In a way, they will be trying to make it as normal a wedding as possible. So when they look around they will both see their families." Guests will be allowed to have their mobile phones with them but they'll have to be switched off as using them would be frowned upon. "A royal wedding is the only time that the Queen isn’t the last person to walk into a room – she has to wait for a bride," Larcombe said. "Once the Queen is there, guests will be on their best behaviour. Things like tweeting will be an absolute no-no. It's likely to be quite laid back before she gets there." Once the Queen does arrive, guests are advised not to talk to her unless they're spoken to but they must acknowledge her if she does look at them. "Gentlemen are supposed to bow, ladies curtsy – guests would need to practise that – but only if the Queen has caught your eye."