I want to select a group of vertices without selecting the ones behind.

Often, selecting vertices that are opposite one another is very useful, such as when you're creating a symmetrical sculpt, but other times you want to be able to select the points on one side of an object without worrying about selecting the ones in back.

There's a button on the 3D View menu called "Occlude Background Geometry." It's marked in red in the image above.

If your screen is split, then this button may be hidden from view. Place your cursor anywhere on the menu, then click and hold your mousewheel and drag to the left until you see it. Click it and your mesh will appear more solid, hiding the vertices in the back of your mesh.

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