For proper horizontal ventilation, the bottom of the windows should be opened on the ______ side.

For proper horizontal ventilation, the correct choice is to open the bottom of the windows on the windward side. This practice leverages the natural flow of air to enhance smoke and heat removal from the structure. When windows on the windward side are opened, fresh air enters the building while pushing smoke and heat out through openings on the leeward side, thereby improving airflow and increasing the effectiveness of the ventilation process.

By directing fresh air into the space, firefighters can reduce backdraft potential and create a safer environment inside the structure. This technique is crucial in fire suppression efforts, as it allows for better visibility and conditions for firefighting operations. Proper ventilation not only aids in extinguishing the fire more effectively but also reduces the risk of flashover and protects both victims and firefighters.