Arches function like doors, except that they do not have a panel which can be opened and closed. In effect, an arch is a door that is always open, and cannot be locked or closed. In fact, many arches are simply door frames without the doors. This is particularly true in The Sims 2, where almost all Maxis-made doors have a corresponding arch.
Most arches are typically one-storey high. The Sims 2: University introduced an arch that is two storeys high, but can only be used from the storey it is placed on.
Despite appearances, an arch appears to work as well as a door for separating one room from another, or separating the indoors from the outdoors. In addition, privacy settings apply to arches just as well as they do with doors. As such, an arch will still be able to keep weather out of the house, or provide privacy for Sims in the bathroom.