Dancers may tie their state of undress to the number of songs played. This could correspond to a bikini performance for song one and topless for the second. In a fully nude club, the sequence could be topless-to-nude over two songs, or any one of a number of other variations. In the United States, clubs are classified based on typical performances, zoning regulations, and advertised services. Zoning regulations in the U.S. can prove a challenge, with clubs needing to be a certain distance away from schools and other areas where minor children are known to congregate. These restrictions are meant, ostensibly, to curb exposure to activities that could negatively impact social development of children and to protect nearby real estate properties not tied to the industry from devaluation. These are known as "secondary effects". The validity of secondary effects has been disputed, with the counterargument that clubs have been forced through zoning into "seedy neighborhoods". Elsewhere in North America, the Zona Norte red light district in Tijuana, Mexico has a number of legal brothels which are modeled on strip clubs and feature U.S.-style striptease performed by its prostitutes.