Which tool creates a concave appearance in mortar joints?

The correct answer is the round jointer, which is specifically designed to create a concave shape in mortar joints. When masons apply mortar between bricks or stones, they often use a round jointer to finish the joints by pressing the tool into the mortar, resulting in a smooth, rounded surface. This type of joint provides both aesthetic appeal and practical benefits, as the concave shape can help direct water away from the masonry, reducing the risk of moisture damage.

In masonry work, the use of a round jointer is essential for achieving the desired finish; it also contributes to the structural integrity of the wall by minimizing the exposed surface area of the mortar, which can be more susceptible to deterioration due to weathering. The specialized design of the round jointer ensures that the finished joint not only looks aesthetically pleasing but also performs well over time.

In contrast, while a trowel is a versatile masonry tool used for applying and shaping mortar, it does not specifically create a concave appearance. A chalk line serves to create straight reference lines for alignment, which is essential in layout work but irrelevant to finishing mortar joints. A masonry saw is used for cutting bricks or stones and would not be suitable for shaping mortar joints.