Until the late 17th century most doors were simply vertical planks held together by wood nailed across the back of them - like a modern shed door.
By 1700 the panelled door was coming into use. This involved a frame in which thinner panels of solid wood were inserted. The frame had two vertical styles (the hanging style held the hinges and the closing or lock style held the handle and door catch). The styles were connected by rails, which instead of being nailed across the back of the styles, were connected by mortice and tenon joints The top rail and the bottom rail were the main members connected the styles. This frame of styles and rails held thinner panels which were slotted into grooves in the frame before the frame was fixed together. The panels were left loose so that they could shrink or expand without splitting. Depending on the number of panels there might be a middle or lock rail and a frieze rail between them.
In Georgian times, The panels were often raised in the centre so that they were flush with the frame, and curved down to the narrower edges necessary for slotting into the frame. Early Georgian front doors almost always had six panels and their proportions adhered to strict Palladian principles appropriate for the facade of a building. So the largest panels would be in the middle (corresponding to the piano nobile of a Palladian villa), the next largest would be at the bottom (where the entrance floor of the villa would be) and the smallest were at the top (where the servants' quarters would be). The internal doors of Georgian houses might sometimes have four or two panels only. The classical rules of proportion required doors to be slightly more than twice as high as they were wide.
Regency doors normally only had two or three panels, but with more pronounced mouldings than before. The panels themselves might have inset or raised circles or diamonds.
Most Victorian doors had four panels and the panels themselves were usually flat.
Doors were only left as bare wood if the most expensive material, such as oak or mahogany was used. It was then polished with beeswax. Most doors were made of pine and they were always painted, whether they were inside or outside. Usually they were painted a dark colour while the door surround was painted white or stone. Inside the house, the dark colour matched the skirting boards which were often brown. Doors might also be grained to imitate expensive woods.
Although fanlights appeared above doors in some Georgian houses, the front door itself was never glazed. In the Victorian era, glass sometimes appeared in the doors themselves. Usually only internal doors were glazed. (If a Georgian door has glass in it, it is because wooden panels were subsequently replaced with glass, by the Victorians or their successors.)