Arbor knot is used to attach the main line to the spool of a reel. This knot can be used for centrepin and fixed spool reels.
Tie a overhand knot in the end of the line. Moisten the knot before you pull it tight and cut the tail off short. Next tie a overhand knot around the main line a few inches up, then moisten and tighten. You should now have a loop of line to slip over the spool as shown above.
Pass the loop of line over the spool and pull the line to tighten the loop down onto the spool. A little pulling back and forth might be required to close the loop down tight.
Cover the knot with a little electrical tape or masking tape. This stops the line slipping on the spool and subsequent layers of line catching on the knot as you fish.