Life lines, are well, a life saver. If you are starting to dip your feet into the knitted lace ocean than I highly suggest knowing how to use a life line. A life line allows you to rip back your work with out losing stitches. They can provide a bit of piece of mind while you are knitting lace for the first time, or are knitting up a complicated lace pattern (like this one).
You will need a tapestry needle and some waste yarn. I highly recomend using a yarn that will not catch or stick to the yarn in your project. Some mercerized cotton is usually a safe bet.
Thread your needle and than slip the needle through the stitch of your work. I like to do it on the live stitches because they are all lined up and easy to see. I also like to install my life line before I work my purl row. If you are using any stitch markers do not thread the life line through them.
Here you can see my life line hanging out either end of my knitting. This is what an life line looks like as you work. Keep knitting your project. Now here is where the life line becomes important. Lets say you notice your stitch count was off, or that you have been doing the lace chart wrong. Have no fear.
Pull your work off your needle and start frogging you work. I know it sounds scary, but I promise that the life line will keep you afloat.
In this photo I have frogged back to my life line. In all actuality I can't frog any farther unless I remove the life line. The life line is acting like a large stitch holder holding all of my life stitches in place.
Now you just pick the stitches back up with your needle and continue on your way. I leave my life lines in till I have finished my project.
Here you can see my sample bound off with the life line still in the work. Easy peasy, just pull one end of the life line and it will slip right out.
Ta Da! Finished.