In masonry chimneys, adjacent flues are usually separated by what?

In masonry chimneys, adjacent flues are typically separated by a wythe of brick. A wythe refers to a single vertical layer of masonry that is one unit thick. This separation is crucial for safety, as it helps prevent the potential for heat transfer between flues, which could lead to chimney fires or the compromising of flue integrity. Proper separation reduces the risk of combustion or damage due to exposure to high temperatures.

The other options may pertain to different aspects of chimney construction but do not provide the same structural function as a wythe of brick. An air space might seem like it could offer separation, yet it typically lacks the necessary structural integrity needed for flue separation. Clay tile liners are generally used for lining individual flues for durability and smoothness but don't separate adjacent flues. Stainless-steel dividers are uncommon in traditional masonry construction and do not contribute to the classic look and function of brick masonry. Therefore, a wythe of brick is the most appropriate choice for separating adjacent flues in masonry chimneys.