WINSLOT is a patented pre-stressed concrete window system designed to offer a high degree of solar control, through the provision of thermally isolated horizontal ‘slats’ which double as lintels and window sills. WINSLOTS may be built into brickwork allowing open slots between each, in height, equal to a chosen number of brick courses. The distance between WINSLOTS should be designed to take into account the prevailing sun angles, to effectively control the penetration of sunlight. Each WINSLOT also becomes a light shelf to bounce light into the interior off the top surface. After the WINSLOTS have been built in as part of the wet-work process fixed glazing or opening windows may be fitted in different combinations to control the degree of ventilation. The opening windows are available in sliding, top hung or central pivot format and may be factory fitted with burglar bars. A recess on the underside of the internal half of the WINSLOT is designed to receive roller or Venetian blinds. WINSLOTS may also be more conventionally used as window sills and lintels to receive larger windows.