contributing factor to the emergence of the Marian diptych. Formats Although the Netherlandish artists are primarily known for their panel paintings, their output includes a variety of formats, including illuminated manuscripts, sculpture, tapestries, carved retables, stained glass, brass objects and carved tombs. According to art historian Susie Nash, by the early 16th century, the region led the field in almost every aspect of portable visual culture, "with specialist expertise and techniques of production at such a high level that no one else could compete with them". The Burgundian court favoured tapestry and metalwork, which are well recorded in surviving documentation, while demand for panel paintings is less evident – they may have been less suited to itinerant courts. Wall hangings and books functioned as political propaganda and as a means to showcase wealth and power, whereas portraits were less favoured. According to Maryan Ainsworth, those that were commissioned functioned to highlight lines of succession, such as van der Weyden's portrait of Charles the Bold; or for betrothals as in the case of van Eyck's lost Portrait of Isabella of Portugal. Religious paintings were commissioned for royal and ducal palaces, for churches, hospitals, and convents, and for wealthy clerics and private donors. The richer cities and towns commissioned works for their civic buildings. Artists often worked in more than one medium; van Eyck and Petrus Christus are both thought to have contributed to manuscripts. Van der Weyden designed tapestries, though few survive. The Netherlandish painters were responsible for many innovations, including the advancement of the diptych format, the conventions of donor portraits, new conventions for Marian portraits, and, through works such as van Eyck's Madonna of Chancellor Rolin and van der Weyden's Saint Luke Drawing the Virgin in the 1430s, laying the foundation for the development of landscape painting as a separate genre. Illuminated manuscript Before the mid-15th century, illuminated books were considered a higher form of art than panel painting, and their ornate and luxurious qualities better reflected the wealth, status and taste of their owners. Manuscripts were ideally suited as diplomatic gifts or offerings to commemorate dynastic marriages or other major courtly occasions. From the 12th century, specialist monastery-based workshops (in French libraires) produced books of hours (collections of prayers to be said at canonical hours), psalters, prayer books and histories, as well as romance and poetry books. At the start of the 15th century, Gothic manuscripts from Paris