The host is seated at the centre of the table, and as a general principle, guests radiate out from the centre of the table in order of precedence.
The principal guest is placed on the host's right. Traditionally the principal guest's wife would be placed on the host's left, the host's wife being placed on the right of the principal guest. If wives are present, the second most important guest would be placed on the host's left.
It is now as likely for the host, or the principal guest, to be a woman, in which case the same basic principles may be applied, with any necessary adaptations employed to achieve the desired balance.
At an official function important members of the governing body should be interspersed among the principal guests. Guests' partners should be placed according to the precedence of the guest invited in their own right. It is up to the host to decide whether husbands and wives are to be seated together or apart. The former is easier to arrange, but the latter (which is always followed at private functions) gives both husband and wife a chance to meet new people.
At a private function the host is customarily seated at one end of the table, and the hostess at the other. Alternatively, the host may be seated in the centre of one side of the table and the hostess immediately opposite him. Exceptionally, if the party is eight in number, the hostess may give up her place to the chief male guest, otherwise it is impracticable to keep to the rule of seating the sexes alternately.
If a party numbers six or ten the table is easier to arrange by placing men and women alternately, but today this custom in not too rigidly applied.
In general, couples are not seated together; traditionally, the exception was those engaged to be married or in their first year of marriage.