Hanging fat stuff

I can't figure out what these are or how they work. That's why Amazon does so well. Best set of pictures of any webpage.

Tbe bedroom set my parents bought me when I was a little boy came with a large, wood-framed mirror.

When my mother moved, she hired a mirror company to hang the mirror.

She showed them the L or J shaped brackets that had gone under the mirror in the two previous homes she had and they said they didn't need it.

A few hours after they left, the mirror fell off the wall.

Broke the glass and the frame. They replaced the mirror but did a rotten job reassembling the frame.