Weddings, celebrations and parties involving sit-down dining require that the guests know where they are seated. In most circumstances, the bride and groom or host designate seating arrangements. Usually, a place card is placed on the table where the guest is to sit.
Place cards keep confusion to a minimum and allow certain people to be grouped together, such as husbands and wives.

Position:
Place cards are usually put at the top of the plate on the table, but they can be laid on top of the napkin
Spousesnner in which it is addressed, should set the tone and give the recipient a sense of how formal or informal the celebration will be.