Quick answer
Seat the people closest to you closest to you: parents, grandparents and the wedding party first. Keep couples together, seat everyone with someone they know, and separate guests who do not get along without making it obvious.
Closest to you, closest to the front
Parents, grandparents and siblings sit at the tables nearest the head table or sweetheart table. Close friends come next, then extended family and everyone else. Guests read the room by distance, so this order avoids hurt feelings.
Divorced or separated parents
If parents get along, they can share a table. If not, give each parent their own table with their own family and friends, at the same distance from you. Nobody should be seated where it looks like a demotion.
Couples and plus-ones
Seat couples together, including plus-ones you have never met. At a long table, put partners side by side or directly across; at a round table, side by side. Splitting a couple across tables leaves at least one person sitting with strangers.
Singles and the singles table
Avoid a table made up only of single guests unless they already know each other. Seat single guests with their friends or with people who share something with them.
Children
Seat young children with their parents. Older children can have their own table if it is near their parents and has something to keep them busy. Ask parents what works for their children.
Guests who do not get along
Seat them at tables far enough apart that they do not need to interact, but not so far that one feels pushed to the edge. Brief someone you trust, perhaps a member of the wedding party, to keep an eye on things.
Frequently asked questions
Who sits closest to the couple at a wedding?
Parents, grandparents and siblings, then the wedding party's partners and close friends. Everyone else follows by closeness.
Where do divorced parents sit at a wedding?
At the same table if they get along. Otherwise at separate tables, each with their own family and friends, at the same distance from the couple.
Should we have a singles table?
Usually not. Seat single guests with friends or people they share something with, unless a group of single friends already knows each other.