Threads are always worked in pairs, as the two ends of each pattern thread laid in with the weft. Often these are worked closely together, but sometimes the two ends are coiled more independently.
One thrima stitch looks like cross-stitch, and is formed by crossing the two thread ends over each other, and up and behind the warp threads. This is used to form vertical lines, and is worked on alternate rows to keep it straight:
Usually partnered with the first stitch in zig-zag designs is a horizontal thrima stitch that looks like chain stitch. This is formed by coiling both thread ends around the next warp threads to the left or right, and is usually worked on the same alternate rows as the vertical cross-type stitch. The technique is one of rolling the two threads between the thumb and forefinger over and around the warp threads being held up by the pickup stick in the other hand. It is fiddly at first!
Another coiling stitch is used for diagonal lines and involves coiling each thread end independently around different warp threads. This can make a thin line that is nice for outlining , sapma shapes, and is worked on every row. For a thicker diagonal line, the thread ends can be coiled around adjacent groups of warp threads to produce two parallel thin lines very close together.
Occasionally vertical lines are also produced by coiling both thread ends together in the same direction around the same warp threads, worked on alternate rows. This is often subsituted for the cross stitch to speed the work when making fabrics for the tourist handicraft market.
The final thrima stitch that I have encountered (so far) looks like satin stitched blocks of colour. The thread ends are coiled around the warp threads at the edge of the desired shape from the front first, then through the shed, emerging in the middle of the shape. This is also worked on every row and looks best on shapes with diagonal sides.
Thrima lines and shapes are often combined with Sapma motifs to produce complex designs, or “flowers” as I hear Bhutanese call them.