An academic library is a library that is attached to a higher education institution, which supports the curriculum and the research of the university faculty and students. According to the National Center for Education Statistics, there are an estimated 3,700 academic libraries in the United States. Class reading materials, intended to supplement lectures by the instructor and housed in academic libraries, have historically been known as "reserves". Before electronic resources became available, the reserves were supplied as actual books or as photocopies of appropriate journal articles. Modern academic libraries provide access to electronic resources.