The frame for a vent hole in a chimney is called a:

The term "thimble" refers specifically to the frame used around a vent hole in a chimney. A thimble is designed to support and provide a transition between the flue pipes or venting system and the masonry of the chimney. It plays a crucial role in ensuring that smoke and gases from a heating appliance are directed safely up the chimney while preventing any potential leaks around the connection point.

In contrast, a saddle typically refers to a roof structure or framing element that diverts water around a chimney or other roof protrusion, while a cricket is a similar feature designed to shed water away from the chimney area but is not associated with the vent hole itself. The term "vent frame" is not a widely recognized term in this context, further reinforcing the specificity of the term "thimble" for this application. Therefore, recognizing that "thimble" correctly identifies the component that frames a vent hole in a chimney is essential for understanding the elements involved in chimney construction and venting systems.