Heckling (or "hackling") splits and straightens the flax fibers, as well as removing the fibrous core and impurities from flax. Heckling is done with heckling combs by pulling the flax through the combs. This parts the locked fibers and makes them straight, clean, and ready to spin. After heckling, the flax is ready to spin. After spinning, the flax is then suitable for weaving into linen.
Heckling is the last of three steps in dressing flax, or preparing the fibers to be spun. Dressing consists of three steps: breaking, scutching, and heckling. The breaking breaks up the straw, then some of the straw is scraped from the fibers in the scutching process, then the fiber is pulled through heckles to remove the last bits of straw.
The process of heckling involves pulling the flax through several different sized heckling combs (or hackles), progressing from coarser combs with only a few prongs or nails per inch, to finer combs with more nails per inch. Generally around three heckling combs are used, although many more can be used. The finer the final heckling comb, the finer the yarn spun from that flax can be. An example of a progression of five combs is first using a heckling comb with 4 nails per square inch, then one with 12 per inch, then 25, next 48, and finally 80 nails per inch. The first three remove the straw, and the last two split and polish the fibers. The finer, shorter fibers that come off in the last hackles is called tow. Tow can be carded like wool and spun, or spun like the other flax fibers. Tow produces a coarser yarn than the fibers pulled through the heckles because it will still have some straw in it. While this yarn is not suitable for fine linens, it can be used for bagging, rough sheets, cords or ropes.
The term "heckle" is thought to be derived from Old English, with Middle English forms hechele, hetchell (c1300), hekele (c1440), hakell (1485), and later hatchel. The terms "heckle," "hackle" and "hackel" are used interchangeably at present.