It is more often than not a Windsor style armchair with a circular or oval pad or tablet replacing the right arm or mounted above it.
On some versions the pad is on a hinge, in order to fold it down and facilitate storage, or simply take up less space in a room.
In this case it is often called a drop leaf chair, and becomes a close cousin to the mechanical desk.
Unlike the antique form, the arm with no pad is usually completely absent, to ease entry in the chair in the crowded conditions of a schoolroom or lecture hall.
Modern designers have offered several contemporary renditions of the writing armchair or the drop leaf chair, but the form has never been very popular in homes.