If you’ve ever followed a family through the census or city directories, and suddenly find that they have moved to a different address on the same street, it may be that they did not move at all, but that the city had changed street numbering systems. As some towns grew, houses or buildings were often just numbered as they were built, counting out from the town center. This became a problem when additional buildings were inserted. Most cities eventually went to a more organized system, usually counting each block as one hundred, and numbering buildings in a correspondingly appropriate position within the hundred and assigning even and odd numbered sides to the streets.
The Sanborn Fire Insurance Maps may be able to help you to determine the corresponding old and new street numbers. When a city was in the process of making such a change, the maps would print both old and new building numbers, so if you encounter such a situation, check to see if Fire Insurance Maps were made for the city. The Genealogy Center has Sanborn Fire Insurance Maps in the Library of Congress, which will help you determine if maps were issued for the city you are searching. The maps will also provide answers if the street name itself has changed, and can help you determine exactly where your ancestor lived.