Price levels in major U.S. cities tend to even out slowly over time. A study of 19 cities from 1918 to 1995 shows that it takes about nine years for prices to get closer to each other. This slow convergence is due to factors like transportation costs, different speeds of adjusting to small and big changes, and including prices of goods that are not traded. This information can help the European Central Bank understand how quickly prices might align in countries using the same currency.