Closed string stairs are staircases where the stringers (the inclined structural supports on either side of the staircase) fully enclose the edges of the threads and risers. 

The edges of the steps are hidden within the stringers, creating a neat and solid appearance. This contrasts with the open string stairs where the threads are visible from the side. 

They are typically made from wood, metal or concrete, depending on the style and setting of the staircase. 

They have a traditional and formal look, though they can be adapted for modern or contemporary styles with clean lines and minimalist materials.

Because the steps are enclosed, they provide extra structural strength.

These stairs are widely used in residential homes, particularly in classic or traditional designs but can also fit seamlessly into modern interiors.