To avoid having to pick up the required warp threads to create each row of a supplementary weft pattern, Lao-Tai weavers have devised several methods of storing these patterns on their loom.

The system allowing the most pattern rows to be stored in the least space can be seen at far left in the drawing by Corinne Matesich below. The weaver has her hand on the reed in the beater, and behind that are the two fixed heddles that are used to create the ground weave. Behind that, a sword is holding the pattern shed open, and at far left is the pattern storage system of long string heddles.

From the weaver’s seat, we can see that this pattern storage system is a mesh of horizontal and vertical strings.

Each heddle consists of one pair of vertical string loops that overlap to contain each warp end. As any one of these vertical string loops is raised, it raises the corresponding warp end:  

The weaver raises a different group of heddles and their warps for each row of the pattern:

As she lifts them, she slides the sword into the shed this creates, and turns the sword on end to hold that shed open while she inserts the supplementary pattern weft thread(s):

To lift the correct group of vertical pattern strings in each row, those strings are separated from the others by being brought forward with a horizontal string that has stored that row of the pattern in the vertical heddle. This horizontal string weaves in & out of the vertical heddle strings in exactly the same pattern that will be created with the weft & the warp, and is simply being transferred from the vertical storage system to the actual weaving, row by row. So each horizontal string in the pattern storage system stores one row of the pattern, and is held in place by being looped around nails in a frame:

Here she has taken the next horizontal string down from her pattern heddle and has swept it back & forth to bring to the front the vertical string heddles she will lift for the next row of her pattern:

Then she replaces the horizontal string loop below the warp (looping it onto posts on each side) to store it in order:

Next she lifts the heddles that have been brought to the front in batches, bringing up with them their corresponding warp ends, and inserts the sword in the shed that is created:

Leaving the sword flat, she pushes it back so she can use the fixed heddles to open the next ground weave shed:

After throwing the ground weave pick, she turns turns the sword to open the pattern shed and inserts the pattern threads to recreate the pattern in her fabric.

Then she repeats the process to transfer each row of the pattern from the pattern heddle storage system to the warp.

This video of the whole sequence begins & ends with the weaver inserting the pattern threads into her weaving:

As Carol Cassidy explains near the end of the following video, the weaver does not have to follow the entire sequence of the stored pattern slavishly. She is free to use sections of it, skip sections, repeat sections, and to mirror sections. In reality, there are two stages of creativity: the first when the master template is created, and the second according to the weaver’s discretion with colours, and which sections to repeat or skip. The same template can be woven with a single pattern colour (kit) or with several colours (chok) and both uses can sometimes be seen in the same weaving.

To mirror the pattern, the weaver takes the next pattern string stored below the warp, sweeps it back & forth to separate the vertical string heddles, and then re-inserts it above the warp and stores it by hanging it on nail in a frame above the warp. This mirroring can be seen in the teal and white weaving further down this post.

Because the pattern storage system, or template, is a sort of weaving in itself, it is possible to see the pattern in it when looking closely. In the pattern storage heddle below, you can see the cross-hatched lozenge-shaped bodies of two siho (mythical lion-elephant creatures), and the head and eye of the one on the left is also quite visible:

Here is the pattern heddle system that is used to weave the teal and white design in the 4th image down:

Below is the beginning of a new pattern storage system. Only the vertical loops are in place. 

To see how a weaver adds the horizontal loops to a new storage system, visit Making a New Pattern Heddle