How can you know that the pound used to weigh your apples is the same whether you are in New York City or Los Angeles?

The weights and measures officials in each U.S. state use standards that are checked against the national standards in Washington, D.C. The standards are kept at the National Institute of Standards and Technology (NIST), an agency of the U.S. Department of Commerce. The NIST (previously known as the National Bureau of Standards between 1901 and 1988) was established by Congress as the central measurement laboratory of the federal government. So no matter where you are in the United States, 16 ounces will always equal 1 pound; 8 fluid ounces will always equal 1 cup; 2 cups will always equal 1 pint; 2 pints will always equal 1 quart; and 4 quarts will always equal 1 gallon.