This section is from the book "The Manufacture Of Boots And Shoes: Being A Modern Treatise Of All The Processes Of Making And Manufacturing Footgear", by F. Y. Golding. Also available from Amazon: The Manufacture Of Boots And Shoes.
There are two types of stitches - the single-thread or chain-stitch, or the double-thread or lock-stitch.
With the former kind of stitch the thread* is deposited in the work with the least injury, and the seam made is elastic, even with such a material as cotton. The appearance of the stitch is different on the two sides of the work stitched. Fig. 192 shows a sketch of a chain-stitch of the twisted-loop variety. A is a section of the material and stitch, B is the upper or right side, while C is the under or twisted-chain loop.
* This term includes silks, cottons, and threads proper.
The lock-stitch is formed by the interlocking of the under or second thread with that of the upper thread. The value of this class of stitch depends upon the accuracy with which the interlocking point has been adjusted to fall in the centre of the material. Fig. 193, A, shows a sectional view of the lock-stitch, the two threads intertwining as near the centre as possible; Fig. 193, B, gives the appearance of the top and underside of the stitch.