Purpose of a trench

[curious]

Looking at (2) channel set, round stone, all around, anniversary bands.

The only difference in them is that one has a trench in the setting and the other does not.

What purpose, if any, does the trench serve? Does it add to or take away from the brilliance of the round diamonds?

Thanks.

 

[Garry H (Cut Nut)]

What is a trench?