Some royal wedding guests are reportedly not being allowed to bring their partners, ruffling a few feathers. But why is the plus-one issue always so tricky?
Two simple words, one almighty headache.
"Plus one" is never added to a wedding invitation, or left off, as an afterthought. For the bride and groom it will probably have involved hours of thought, several more hours of heated debate and a couple of blazing rows.
For a guest it can be an equally big headache, as some of those invited to the royal wedding are finding out. Newspapers have reported that hundreds did not receive a plus one from Prince William and Kate Middleton and it has left their partners fuming.
But for any bride and groom, regardless of a royal title, the dilemmas involved seem endless. Do you invite a friend's partner if they are married but you don't really know them? What if they've been together for years but you don't like the other half? What if your friend gets through partners at a rate of knots? Will they come if they can't bring someone?
The authority on etiquette, Debrett's, says traditionally, if you were not known by the bride's mother, you did not get an invitation.
Now, while there is no generally accepted rule, if the guest is married or in an established, long-term relationship, his or her other half should be invited. But family should always come before friends.
It can be big decision as some people take it very personally. Adam, 32, who does not want to use his real name, lost a friend over a lack of plus one. He and his partner did not invite his best mate's girlfriend.
"In the end my best friend did not turn up to the wedding either and the friendship never survived," he says.
Plus one is a relatively modern dilemma, says Liz Brewer, etiquette expert on ITV's Ladette to Lady. It's only over the last century that weddings have grown into such big affairs and it has become customary for guests to bring someone with them. Before they were smaller and more personal, so plus ones were not really necessary.
She says if you do get a plus one you can take anyone, not only a partner, but - and it's a very big but - the guest has to be appropriate for the occasion.
"Not someone who drinks too much or is a party bore. Not an ex of the bride or groom and preferably not someone who is not on the guest list for a reason.
"Sometimes a thick-skinned wannabe will go to extraordinary lengths to get invited, especially if they know the hosts have for their own personal reasons decided against extending an invitation. I always check exactly who is being brought as the plus one."
For the bride and groom it often comes down to cost and shelling out extra money for someone they might not know very well, if at all. With money tight for many people at the moment, happy couples are having to get tough.
"Weddings are expensive and a plus one can add up to more than £100, which is money most couples would rather put towards something else," says Belinda Hanks, of wedding company Confetti.
"The general consensus is really if they are long-term partners, married or engaged then they are in. Having said this, the consensus for most people is that if it's a plus one for a guest who would otherwise be alone, they will oblige, even if they don't know their partner."
But others are very clear that it's not their job to accommodate other people's personal situations.
"I hate plus ones," says Lucy Mangan, author of The Reluctant Bride. "Friends are friends, they often marry nice people but if you're honest they never really become your friend too. Inviting them just doubles the guest list and often the plus one doesn't even really want to be there.
"People should be grown up enough to say 'I am the friend and I am more than an indissoluble body from my partner'. It's slightly childish and self indulgent if they can't. As for inviting someone you don't know so they can keep someone company, stuff that."
So is there any negotiation on the subject? Not if it's the royal wedding, even if it doesn't mess with the already tight numbers. One royal wedding guest reportedly asked if he could give his invitation to his wife instead. Apparently, they are non-transferable.
But when it comes to the average wedding, is there anything you can do if you want to take a partner? It's debatable. But Brewer says you can politely ask.
"If there is no plus one but you want to bring someone, you politely telephone asking if you can bring a guest.
"Say 'I have a partner now so would it be possible...' or 'I am engaged now so...' Always say you understand if it is not possible."
But David Miller, director at etiquette guide Debrett's, says asking is always a no no.
"Never ask. It is the height of rudeness."
But if you are just horrified at the thought of going to a wedding alone, it's time to grow up, says Mangan.
"We're all old enough to go to a party and mix with people, if you can't it's rather pathetic."