What is the masonry wall or partition separating multiple flues in a chimney called?

The masonry wall or partition that separates multiple flues within a chimney is referred to as a wythe. In the context of chimney construction, a wythe serves as a structural element that helps to compartmentalize the individual flues, preventing heat and gases from one flue from transferring to another. This is crucial for ensuring that each flue operates effectively and safely without interference from adjacent flues.

The term "wythe" specifically relates to the vertical section of masonry that helps maintain the integrity and function of the chimney system. Understanding the components of a chimney, including the wythe, is essential for ensuring proper construction and adherence to safety codes.