Bricks with Han-Nom characters were found a lot in the area of Thanh Nha Ho (Citadel of Ho dynasty). They include two types based on materials and sizes: Vinh Ninh Truong (Vinh Ninh school) bricks and bricks from other locations. So far, over 100 names of hamlets and communes have been found on the bricks indicating their production places that provided materials for the construction of the imperial city during 1397-1403. These locations are mainly some districts of Thanh Hoa province, including Vinh Loc district where provided the most. Vinh Ninh Truong (Vinh Ninh factory) bricks and some other types, which had been supplied by some local areas for the construction of palaces in Thang Long Imperial Citadel previously, were used to build Thanh Nha Ho in 1397. Vinh Ninh factory was actually a manufacturing centre of the Tran - Ho period bricks and can be regarded as a famous brand for brick production at that time.