When stitching book sections together it is not uncommon to have a piece of thread that is too short to complete the job. If this is the case, then you need to join another length to the first.
The traditional technique for joining a piece of thread on is to use a weavers knot, making sure that the knot remains on the outside of the spine. This can be a bit complicated because it is hard to position the join in just the correct position so that when the thread is pulled tight the knot does not get pulled through into the inside of the book. There are lots of different ways to achieve this, and most good books on bookbinding will give you a description.
These can be difficult to get right, however, and when using fine threads tying the actual knot can also be very fiddly. Many binders simply make sure that the thread is long enough for the entire job to start with, but if there are lots of sections the thread can become quite long. This in turn can cause it to knot up by itself, and it takes a long time to pull through each hole.
A much easier technique is to do the following:
Step 1: When the first thread reaches the end of the section, tie it off using a double kettle stitch, just as you would at the end of a job anyway. Leave a short length free to glue down later.
Step 2: With the new length of thread, tie it to the kettle stitch at either end. This can also be done with a double kettle stitch, again with a small length of end left free to glue down later.
Starting at the other end of the block to the one you just did in step 1 leaves a smaller knot, which is a bit better.
Step 3: Complete the sewing of sections as usual.
Thanks to Bernie for this very useful tip.