E/C.12/GC/20 page 4 enjoyment of Covenant rights is often influenced by whether a person is a member of a group characterized by the prohibited grounds of discrimination. Eliminating discrimination in practice requires paying sufficient attention to groups of individuals which suffer historical or persistent prejudice instead of merely comparing the formal treatment of individuals in similar situations. States parties must therefore immediately adopt the necessary measures to prevent, diminish and eliminate the conditions and attitudes which cause or perpetuate substantive or de facto discrimination. For example, ensuring that all individuals have equal access to adequate housing, water and sanitation will help to overcome discrimination against women and girl children and persons living in informal settlements and rural areas. 9. In order to eliminate substantive discrimination, States parties may be, and in some cases are, under an obligation to adopt special measures to attenuate or suppress conditions that perpetuate discrimination. Such measures are legitimate to the extent that they represent reasonable, objective and proportional means to redress de facto discrimination and are discontinued when substantive equality has been sustainably achieved. Such positive measures may exceptionally, however, need to be of a permanent nature, such as interpretation services for linguistic minorities and reasonable accommodation of persons with sensory impairments in accessing health-care facilities. 10. Both direct and indirect forms of differential treatment can amount to discrimination under article 2, paragraph 2, of the Covenant: (a) Direct discrimination occurs when an individual is treated less favourably than another person in a similar situation for a reason related to a prohibited ground; e.g. where employment in educational or cultural institutions or membership of a trade union is based on the political opinions of applicants or employees. Direct discrimination also includes detrimental acts or omissions on the basis of prohibited grounds where there is no comparable similar situation (e.g. the case of a woman who is pregnant); (b) Indirect discrimination refers to laws, policies or practices which appear neutral at face value, but have a disproportionate impact on the exercise of Covenant rights as distinguished by prohibited grounds of discrimination. For instance, requiring a birth registration certificate for school enrolment may discriminate against ethnic minorities or non-nationals who do not possess, or have been denied, such certificates. Private sphere 11. Discrimination is frequently encountered in families, workplaces, and other sectors of society. For example, actors in the private housing sector (e.g. private landlords, credit providers and public housing providers) may directly or indirectly deny access to housing or mortgages on the basis of ethnicity, marital status, disability or sexual orientation while some families may refuse to send girl children to school. States parties must therefore adopt measures, which should include legislation, to ensure that individuals and entities in the private sphere do not discriminate on prohibited grounds.