Wythe

That wall has to be at least three wythes of brick to support your load.

A chimney runs from the base of the first flue liner to the top of the last flue liner, or to any rain cap above it. Attach single-wythe chimneys to the house.

[…] Where there is more than one flue, there is usually a single wythe of masonry separating the flues. Particularly on older chimneys with poor or missing caps, this wythe of brick dividing the flues may be[…]