It is generally accepted that the majority were completed by Jan, from an overall design by Hubert, who probably oversaw the construction of most of the frames. A number of difficulties present themselves, not least that there is no surviving work confidently attributed to Hubert, and it is thus impossible to detect his style. Instead, art historians compare individual passages to known works by Jan, looking for stylistic differences that might indicate the work of another hand. Advances in dendrochronology have for example established that parts of the wing panels were felled around 1421. Allowing a seasoning time of at least 10 years, we must assume a completion date well after Hubert's death in 1426, thus ruling out his hand from large portions of the wings.