Mixed-use zoning allows for the coexistence of residential, commercial, and/or industrial uses in a same district. Mixed-use zoning can take many forms, but it is commonly classified into three types: vertical mixed-use, horizontal mixed-use, and mixed-use walkable. In vertical mixed-use districts, all floors within a building are used for different purposes. Most common example is the use of rental apartments on the upper floors and retail or restaurant spaces down below. The reason why this type of zone is called "vertical" is because all the uses are arranged in an upright position, one above the other. In contrast, horizontal mixed-use zones feature buildings with multiple stories but where each floor serves a single purpose - either commercial or residential. Mixed-use walkable neighborhoods are compact areas with a mixture of residential and commercial establishments located around a central park area or town center. They are usually designated as R1-4 or MC3-5 where "R" stands for "residential" and "C" for "commercial". These are just examples of how mixed-use zones can be implemented. Many cities across the United States have adopted various forms of mixed use zoning over time. Some examples include Baltimore, Maryland; Chicago, Illinois; Denver, Colorado; Houston, Texas; Los Angeles, California; New York City, New York; Philadelphia, Pennsylvania; Portland, Oregon; San Francisco, California; Seattle, Washington.