Piglet, you go by convention.
Convention doesn't happen in a vacuum. It is distilled wisdom passed down from the time people started to celebrate weddings as we currently understand them.
Convention indicates you invite all or none.
If you want 'all' then that means their spouses and partners too.
Above all, convention dictates that you find a venue that offers the two factors of affordability and ability to handle the people you should invite, even if that means you don't get the Pinterest inspired wedding of your dreams.
People here are saying 'weddings cost XXX amount', as if all weddings are prix fixe, take it or leave it, with no room for doing a bit of work and finding affordable options. You can indeed adjust details, as Palomapaloma did.
If you refuse to compromise then it is clear what your priority is, and it is not your guests. People do indeed understand, but probably not in the way you think they do, Strawberrychunk, especially if they are regaled afterwards with Instagram and FB posts that reveal where the wedding budget went.
A wedding of an old school friend is a different matter from the wedding of a family member, Chikara. Were the family members of the bride and groom also invited to the school friend's wedding sans partners or spouses?
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I am speaking as one of 66 first cousins. My mum is one of eight and my dad was one of eleven.
Weddings in my family feature aunts and uncles, with cousins invited under certain circumstances.
To represent families where both the aunt and uncle are dead, the cousin who is the oldest in that family/lives nearest to the venue/closest in terms of friendship to the bride or groom, and their spouse or partner get invited.
If an aunt or uncle is widowed, then one of their children gets invited along with spouse or partner, to accompany the widow/er, drive them to and fro if necessary, and represent the deceased relative. It's usually a case of oldest in their family or offspring living closest to the wedding venue or again, closest in terms of friendship to bride or groom.
A cousin who also occupies a position in relation to the bride or groom such as godparent or someone who practically brought up the bride or groom or a cousin who was always very close is normally also invited, along with spouse, though sometimes those cousins would be best man, matron of honour, or bridesmaids/groomsmen.
After family has been invited, numbers are made up by friends.
This system has always worked well.
One of the best aspects of it is the way it manages expectations of all 66 of the potential guests. Nobody has any expectation of getting an invitation. There are no disappointed cousins' children, no elderly widowed men or women wondering who is going to drive them from the church to the reception or drive them home again... People marrying in have brought their own customs but most have already come to similar arrangements in their own families. Mostly there is a half and half split as far as guests from both sides goes. Without the 'token parties' arrangement for weddings in my family, they would be unmanageable.