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You'll need 2 J brackets for the bottom, with a cradle wide enough for the thickness of your mirror, and 2 sliding J brackets for the top. Use as many hooks as you need to support the mirror's weight. Evenly space them out and install them with sturdy hollow-wall fasteners for wallboard or plaster, wood screws for wall framing, or masonry anchors if you're fastening the hooks to brick or concrete. You may also want to pad the J-hooks with pieces of felt to protect the back of the mirror. You can also use a single J-shaped metal strip called a J-strip. You should pad this piece as well. Use a level and a pencil to draw 2 lines on the wall, indicating the length of the mirror's sides. They should extend upward, both at a right angle to the bottom horizontal line. Following the same intervals as the bottom J-hooks, loosely attach additional fasteners that will keep the mirror in place. Choose L- or Z-clips based on the mirror's design, then screw them into the wall. Keep the screw loose so the front side that will rest against the mirror is loose as well. Since these clips won't support the mirror's weight, you only need light-duty fiber or plastic anchors called wall plugs to install them. Pad the L- and Z-clips with adhesive-backed felt where they'll touch the glass. Move the side fasteners out of the way before setting the mirror's bottom edge into the J-hooks. Lift the mirror by the sides and tilt it backward against the wall, then tighten the L- or Z-clips just enough to hold the mirror firmly.
Use brackets to hang heavy unbacked mirrors. Install a row of 3-4 J-hooks along the bottom line drawn on the wall. Mark where the sides of mirror will sit on the wall. Install 2-3 L- or Z-clips along the drawn vertical lines on the wall. Slide the mirror into place and tighten the clips to keep it secure.