(a) Availability. Functioning educational institutions and programmes have to be available in sufficient quantity within the jurisdiction of the State party. What they require to function depends upon numerous factors, including the developmental context within which they operate; for example, all institutions and programmes are likely to require buildings or other protection from the elements, sanitation facilities for both sexes, safe drinking water, trained teachers receiving domestically competitive salaries, teaching materials, and so on; while some will also require facilities such as a library, computer facilities and information technology; (b) Accessibility. Educational institutions and programmes have to be accessible to everyone, without discrimination, within the jurisdiction of the State party. Accessibility has three overlapping dimensions: Non-discrimination - education must be accessible to all, especially the most vulnerable groups, in law and fact, without discrimination on any of the prohibited grounds (see paras. 31-37 on non-discrimination); Physical accessibility - education has to be within safe physical reach, either by attendance at some reasonably convenient geographic location (e.g. a neighbourhood school) or via modern technology (e.g. access to a “distance learning” programme); Economic accessibility - education has to be affordable to all. This dimension of accessibility is subject to the differential wording of article 13 (2) in relation to primary, secondary and higher education: whereas primary education shall be available “free to all”, States parties are required to progressively introduce free secondary and higher education; (c) Acceptability - the form and substance of education, including curricula and teaching methods, have to be acceptable (e.g. relevant, culturally appropriate and of good quality) to students and, in appropriate cases, parents; this is subject to the educational objectives required by article 13 (1) and such minimum educational standards as may be approved by the State (see art. 13 (3) and (4)); (d) Adaptability - education has to be flexible so it can adapt to the needs of changing societies and communities and respond to the needs of students within their diverse social and cultural settings. 7. When considering the appropriate application of these “interrelated and essential features” the best interests of the student shall be a primary consideration.