The Brahmanical centres of learning were open to all the Dvija castes, states Romila Thapar, and they "appear to have attracted mainly the Brahmin students". The Dvija restrictions may be theoretical, because most students learnt their economically useful skills through apprenticeship within their family or at guilds. Other scholars also treat the Dvija concept as theory, because in reality ancient and medieval India had artisan schools where all social classes including Shudras learnt skills, and these artisans and workers built sophisticated artwork, products and large temples.