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The safest recommendation is to put one in every room, but most people are not willing to do this. The simplest rule is to mount one between the bedrooms and the rest of the house, but closer to the bedrooms. If there is more than one sleeping area, each should have its own alarm. In multi-level homes, install one on each level, and if possible have them interconnected so any one unit will sound the alarm throughout the house. The basement ceiling, near the steps, is a good location for extra protection.