The basic join:
The purpose of the basic join is to integrate the tatting in progress with a piece of tatting previously accomplished. This can be joining ring to ring, ring to chain, chain to chain, or motif to motif.

The basic join uses a free standing picot on a previously tatted ring/chain. The working thread of the new tatting segment is pulled through the picot to make a loop. The shuttle passes through this loop carrying the foundation thread (core or shuttle thread) forward. The loop is then pulled back through the picot and the work is snugged up securely and tatting continues on the new segment.

Consider the shuttle (core or foundation) thread as a horizontal line, the line of progression. The basic join is completed above the line of progression. The basic join can be anywhere in the 180 degree arc above the line of work.

But what if the join needs to be completed below the line of progression, downwards to a previous row/ring/chain? You may have read patterns that ask you to join in a seemingly impossible way. This situation calls for a different join, i.e., the shuttle lock join.

Theory:

One uses the shuttle join to join down instead of up. It is used most often when doing lots of chainwork as in the edging of the Valentine's Day Heart Pillow shown below. The shuttle thread is pulled through a picot or point of attachment in a loop and the shuttle passes through the loop made. Hold the shuttle thread gently and bring the work in progress over so that it touches the picot/point used to join. Then gently pull up the slack in the shuttle thread and tighten.

One usually compresses the stitches prior to making the shuttle join to enhance the curve of the work if desired. Also, be sure to check the correctness of your work before making the shuttle join as it is a real bear to undo.

And concentrate on pulling the shuttle thread straight up at a 90 degree angle when tightening it. Right or left hand tatters often work a slant into the shuttle join by pulling in their dominant direction. Pull straight up to avoid the slant.

Step by step shuttle join basic instructions:

1. When joining to a tatted point below the horizontal line (below the line of progression,) the tatting is accomplished to the point of the proposed join.
2. Snug up the double stitches and check the correctness of the work to that point. A shuttle join is not easily undone.
3. If the work is correct, let the shuttle thread drop down behind the free standing picot or other point of attachment on the previous tatting.
4. Pull a loop of the shuttle thread through the picot, slip the shuttle through this loop, but do not tighten it yet.
5. Pull on one side of the loop only, so that the new tatting is drawn up to the old tatting.
6. Next hold both segments under the thumb while the shuttle thread gently removes the slack in the loop and tightens down.
Continue to tat.