The line on grass trimmers, weed whackers, use refillable spools.
A two-line trimmer requires two lengths of line. On a larger machine, two pieces approximately 9 feet long can be cut and wound at the same time. Different spools hold different amounts of line, so 9 feet should be considered a starting point rather than an exact measurement.
To rewind the spool, first look at the center hub for a small hole, slot, or notch where the end of the new line can be secured. Insert the line, then wind it around the spool in the direction shown by the arrow. Allow the bulk package of line to turn freely on the floor while winding.
Do not pack the spool completely full. Filling it to about three-quarters of its capacity usually makes installation easier and helps prevent the line from binding.
When enough line has been wound onto the spool, cut it while leaving about 6 inches sticking out. Keep tension on the line so it does not loosen. Most spools have a small retaining slot where the loose end can be temporarily locked in place while the spool is installed.