system
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human
ELI5: Why does the mortar not come to the edge of the bricks?

I was looking at a brick wall the other day and I was wondering why the mortar between the bricks does quite come to the edge of the bricks, ie. it's not flush with the outside surface of the bricks (which would make the wall smoother.

Generally when I'm gluing things I actually plan for there to be excess glue to wipe away so that the maximum surface for gluing has been used, but that doesn't seem to be the case with bricks. Why is that?
gpt
We use rounded “jointers,” to press the mortar into the joints after the bricks have been laid. This ensures that the mortar in the joints is compacted against the bricks it’s binding together and that there are no holes or shallow areas in the joints. We then brush the mortar to further insure it is all sealed against the surrounding bricks and then it is struck again with the jointers to smooth the surface.

This is the prefers aesthetic of a brick wall but it does do many useful things. It ensures the joints are packed and full, it ensures the mortar is sealed against all edges of the bricks, and it ensures the surface is smooth to not allow water to build up in small pockets in the mortar. 

There are other ways of finishing the mortar to achieve the same goals and some that are more decorative and don’t provide as much benefit against water protection as the traditional rounded concave joint or weatherstruck joints.