Code selections in North America were based on existing radio station identifiers. For example, radio stations in Canada were already starting with "C", so it seemed logical to begin Canadian airport identifiers with a C (Cxxx). The United States had many pre-existing airports with established mnemonic codes. Their ICAO codes were formed simply by prepending a K to the existing codes, as half the radio station identifiers in the US began with K (with the other half using the letter W, which is used in Southeast Asia). Most ICAO codes outside the US and Canada (except for some airports in Mexico) have a stronger geographical structure.