The plan is made on paper, weeks out
A seating plan is finished a fortnight before the day, not in the last week. Two reasons, both practical: cards have to be printed, and the person making the plan needs enough time to notice what the plan is doing. A table settled in one late evening is usually a table arranged to keep two people apart, and everyone at it can feel that.
The plan is a single sheet with the room drawn on it, every table numbered, and every name written in the place it will occupy. A steward keeps one copy at the door for the evening, because plans change, and the person standing at the entrance with a question should not have to find the couple.
Who sits where, and on what principle
Three principles, in this order. The couple first: together, at the centre of the room rather than at the end of it, so conversation reaches them from both sides. Then the two families, seated in the order they already keep among themselves, because a wedding is a day when people should recognise their own place rather than learn a new one. Then the friends, placed in groups that already talk to one another.
Parents sit nearest the couple, one set on each side where both are present. Grandparents sit where they can see and be seen without turning, and never at the end of a long table where service reaches them last. A guest who came alone sits between two people who will speak to them, which is the one place where a host is allowed to be deliberate rather than natural.
Families that do not agree
Where a divorce, a quarrel or a long silence divides a family, the plan answers it and the day does not. Put distance on the sheet: different tables, or one table with four covers between them, and no explanation offered to anyone. An explanation turns a seating decision into a subject, and a subject is what the room will discuss by the coffee.
If one of the two asks why they were placed apart, the answer is a short, true, dull sentence: the room was arranged this way. It is not a verdict and should not be delivered as one. A host who explains at length has made the guest the topic of the evening, which is exactly what the plan was there to prevent.
The tables beyond the top
The remaining tables are arranged by affinity rather than importance. Seat together the people who will still be talking when the plates are cleared; a table of strangers arranged alphabetically is a table that is quiet by the second course. Children sit beside a parent, or at a table with the older children, and never alone among adults who are not theirs.
Every table wants one person who will draw others in, and that person is not always the loudest: it is usually the one who asks questions. Ten covers is a good number at a round table and eight is better at a rectangular one, and no table should be so long that the far end is a different party.
The card, and the change at the door
Place cards carry the name as the house writes it, with a table number on the reverse where there is no escort card. Cards are set before guests enter and are not rearranged during the meal. If a guest arrives with someone unannounced, add one cover at the end nearest the host and do not reorder the table: reordering tells the room that the plan was a preference rather than a decision.
After the meal the plan is kept. A house keeps its wedding plans with the menus and the letters, because in twenty years that sheet will say who was there and where they stood, and no photograph will ever say it.