The join is a technique used in constructed large pieces of lace from smaller tatted bits. There are many types of join, i.e., up, down, shuttle lock join, onion ring, split ring, swirl, lark's head picot join (LHPJ), and many others.
The Onion Ring Join
Using shuttle one (SH1), begin by tatting a normal ring with a picot at top center. Close ring and do not reverse work. Using the same shuttle begin a second ring and tat up to the midpoint. Next bring the ring thread up through the picot on the first ring. Pull gently on the ring thread to remove the excess thread. Make sure that the last double stitch formed touches the picot; adjust tension or compression as needed. Finish tatting the ring and close allowing the new ring to surround the first ring.
The onion ring join makes it possible to create the look of a ring surrounded by a chain but using only one shuttle. If a second shuttle is introduced then regular chain work can be accomplished and additional rings thrown off the chain using the second shuttle. Additional shuttles can also bring in more color to the piece and the additional threads can be encapsulated in the chains, etc.
The Lark's Head Picot Join
This technique gives the look of a double stitch on the top of a picot where the join occurs. Bring the thread from the new ring/chain up through the picot on the piece to which you are joining. Fold the bend of this loop over, back onto itself in the manner of a luggage tag loop. You will see the two tails on the bottom and now double loops resting on top of them.
Now bring the shuttle thread through these two loops. Go OVER the right hand loop's edge, go UNDER the double tails and exit going OVER the left hand loop's edge. At this point you can clearly see the resemblance to the "luggage tag" loop.
To secure the join in place, gently pull on the side of the loop closest to the last double stitch made. Hold this half stitch firmly while removing the slack from the other side of the loop. When complete, there is a complete double stitch on the legs of the original picot and there is a complete double stitch at the top of the picot, too.
The Inverted Ball Thread Join
Seen often in Dutch tatting patterns, this inverted join uses the ball thread. A chain is being tatted perhaps the start of a leaf. The chain has a few picots placed on it which are graduated in size, the smallest being made last. The chain is then bent to being working back in the opposite direction. The completed chain is position above the work now in progress and the ball thread (SH2) is in the space between the chain segments and the core thread (SH1) lies at the bottom position.
Take the ball thread (SH2) and pull a loop of thread up through the picot from the completed chain. Pass the ball thread (SH2) through this loop. Note that the core thread (SH1) is NOT involved in this join. Remove slack from the ball thread.
The upper picot is now stretched into the open area and is met half way by the ball thread creating the look of two joined picots. You can alter the amount of open space in the middle by pulling more or less slack from the ball thread join. Continue to chain and repeat.