where goods or individuals moving across borders are inspected for compliance with border control measures. Access-controlled borders often have a limited number of checkpoints where they can be crossed without legal sanctions. Arrangements or treaties may be formed to allow or mandate less restrained crossings (e.g. the Schengen Agreement). Land border checkpoints (land ports of entry) can be contrasted with the customs and immigration facilities at seaports, international airports, and other ports of entry. Checkpoints generally serve two purposes: To prevent entrance of individuals who are either undesirable (e.g. criminals or others who pose threats) or simply unauthorised to enter. To prevent entrance of goods or contaminants that are illegal or subject to restriction, or to collect tariffs in accordance with customs or quarantine policies. A border checkpoint at which travellers are permitted to enter a jurisdiction is known as a port of entry. International airports are usually ports of entry, as are road and rail crossings on a land border. Seaports can be used as ports of entry only if a dedicated customs presence is posted there. The choice of whether to become a port of entry is up to the civil authority controlling the port. An airport of entry is an airport that provides customs and immigration services for incoming flights. These services allow the airport to serve as an initial port of entry for foreign visitors arriving in a country. While the terms airport of entry and international airport are generally used interchangeably, not all international airports qualify as airports of entry since international airports without any immigration or customs facilities exist in the Schengen Area whose members have eliminated border controls with each other. Airports of entry are usually larger than domestic airports and often feature longer runways and facilities to accommodate the heavier aircraft commonly used for international and intercontinental travel. International airports often also host domestic flights, which often help feed both passengers and cargo into international ones (and vice versa). Buildings, operations and management have become increasingly sophisticated since the mid-20th century, when international airports began to provide infrastructure for international civilian flights. Detailed technical standards have been developed to ensure safety and common coding systems implemented to provide global consistency. The physical structures that serve millions of individual passengers and flights are among the most complex and interconnected in the world. By the second decade of the 21st century, there were over