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Find existing wall fixtures. Tie on your string. Hang your pictures.
Look for hooks, screws, vents, or knobs that are already in place and that could bear a few extra pounds. Note that this method will be most appropriate for lightweight pictures that aren’t in frames. Look for wall features that are out of the way and that could have a string run between them without posing a strangulation risk. Cut a length of string, twine, or wire that’s long enough to span the distance between the two wall fixtures, plus some extra for tying knots. Tie each end of the string to a separate wall fixture. You can either pull the string taut or leave it slightly loose and droopy.  A taut string will look more rigid and uniform, while a droopy string will look more relaxed and artistic. The choice between the two is a matter of aesthetic preference. Wire will be harder to tie than string or twine (it will be easier to wrap it tightly around your fixture than tie it), will provide an industrial look, and will allow pictures to slide around for quick re-positioning. Wire is thin and strong, but doesn't lend itself to the droopy look. Twine and string will be easy to tie, can be left droopy or pulled taut, and will provide a more rustic look.Twine will be thicker than wire or string, but stronger than string. String will be thinner than twine, but not as strong. Use clothespins or clips to affix your pictures to the string. If your string begins to sag more than it's supposed to or the knots won't stay tied, you may have too much weight. Either use stronger twine or wire, or tie a second string to different fixtures for a second row of pictures. To distribute weight and pictures evenly, place your first picture in the center of the string using your eye or a tape measure. Using the first picture as the center point that separates two halves, divide those halves in half again, and place one picture at each center point. Continue dividing your spaces in half and using the center points as picture locations until all your pictures are hung.