Seating Guests used to be easy - one just followed the rules and everything went smoothly. But what if you have chosen a park for your wedding? Or how should a couple seat their parents if they are divorced? How should one seat Guests for interfaith weddings?
Even if your wedding is in a park or some other non-traditional spot, you may still want to have an aisle and a processional. If so, then that structure is enough to have a front "row" for honored Guests, such as your parents. Traditionally, the front row is reserved for immediate family (parents and siblings). The Mother of the Bride and the Mother of the Groom get to sit on the aisle. If the parents are divorced, usually the Father will sit in the second row, on the aisle, with the Grandparents.
Readers and singers should be seated next to the aisle so they will be able to get up without disturbing anyone else.
Traditionally, in a Christian ceremony the Bride's side is on the left and the Groom's side is on the right (when one is looking at the altar). The way to remember this rule is by thinking of the old joke, "This is the last time the Groom is right". In a Jewish ceremony, the sides are reversed, and so in an interfaith ceremony couples often choose to let Guests sit wherever they wish.
Ushers usually hand out programs, and if a single woman arrives without a partner, one of the Ushers escorts her to a seat. Usually single men are selected as Ushers for this very reason. If there is an aisle runner, the Ushers unroll it from the altar to the entrance, just before the Wedding Party enter.
The last Guests to be seated are the Grandparents, then the Parents. The order alternates, with the Groom's side first. So, if just the parents were seated, the Father and Mother of the Groom would go first, then the Mother of the Bride with an Usher. In Jewish ceremonies, the Groom ushers in his parents, and many modern wedding ceremonies, even those that aren't Jewish, have started using that tradition, as well.
It's worth actually practicing the seating of the Parents during the wedding rehearsal, so everyone is comfortable and confident about what to do on the day of the wedding!