Which type of window provides only a 50% opening of its total area?

Horizontal sliding windows are designed to slide open horizontally, with one panel moving past another. This type of window allows for an opening that typically covers only half of its total area at any given time, as one side remains fixed while the other can slide open. This design can be advantageous for ventilation without compromising security, making it a popular choice in both residential and commercial settings.

In contrast, casement windows open outward using hinges on the side, often enabling a full opening of the window area for maximum airflow. Skylights are generally fixed above and allow light in, but they usually do not provide any significant opening for ventilation. Bay windows comprise multiple sections and often do not function as traditional windows; instead, they serve more for aesthetics and extended views rather than providing a specified percentage of operability. The design and functionality of horizontal sliding windows make them unique in providing only a 50% opening of their total area.