This rig has many advantages, the first being that it combines both the subtlety and hooking potential of a braid and the invisibility of fluorocarbon. This is a rig which will help you catch many species of fish but carp and tench will fall to this rig the most often.
For the braid part we prefer to use kryston merlin and for the mono part 15lb Pro Gold. The image shows the rig with a double boilie bait presentation.
To create the join, firstly tie your hook to the braid as you would normally do, using the knotless knot you then create your hair. Your finished braid section should be around 5 - 6 inches long.
You are now ready to connect it to the fluorocarbon. Create a loop in the Fluorocarbon and hold it there. Poke the braid through the Fluorocarbon loop that you have created. Now whip up the Fluorocarbon rotating over it six times. Whip back down the Fluorocarbon twice. Then take the supple tag end and go back through the loop up to the hook. Moisten the knot and slowly pull it tight. Then, trim the tag ends. Roll heavy metal putty or similar around the knot and you have now created a combi rig.