With R1 zoning, the lot is defined with specific boundaries, and no mobile homes or multi-family units are allowed. Local ordinances differ, but almost invariably specify maximum height, lot size and distance of setback from the street and adjacent houses. The purpose of setbacks in residential zoning is to reduce the level of noise, protect residents from vehicle traffic and create barriers so fires do not jump from one home to another. Some R1 zoning even requires garages or carports to prevent vehicles from clogging residential streets. In other areas, street parking is restricted to one car per residence. With an R1 rating, the "R" refers to residential, while the "1" indicates the limitation of one home or structure. It normally follows that an R2 rating allows for two residential dwellings, typically in the form of a duplex, while an R3 permits multi-family units such as apartments or condominiums. Commercial use of an R1 property is generally not allowed, although home offices or professional businesses such as a law firm or a dentist office are sometimes permitted.