A cabled yarn is simply a plied yarn that is plied back on itself. In its most basic form, a cable yarn is 2 - 2 ply yarns which are them plied together. Easy enough to accomplish.....or so you’d think. Yet, simply plying two 2-ply yarns together will not give a good cabled yarn.

The key to making a good cabled yarn is twist and understanding the direction of the twist.
Let’s call clockwise twist Z and counter clockwise twist S.
Step one is to spin two bobbins of Z twist singles.

Step two is to ply the two bobbins of singles together using S twist but with extra plying twist.
Depending on the fibers used and how soft or stiff of yarn you desire, add from 15 to 30 percent more twist than would be needed to ply a balanced skein. Yes, we are deliberately over plying.
Do not skein the yarn off of the bobbin.

Step three is to spin two more bobbins of Z twist singles and to ply them together S, being careful to add the same amount of extra twist as you did to the fist bobbin of plied yarn.

Step four is to ply the two bobbins of plied yarn together using Z twist. The extra S twist inserted into the two 2 ply yarns is removed when the two 2 ply yarns are plied together Z.
As you ply, stop frequently and check to see that your yarn is not under or over twisted at this point. Pull a loop of the yarn back through the orifice of your wheel and let it hang down. It should hang in a straight loop.

Cabled yarns are stronger than plied yarns because they have a higher degree of twist energy binding them together. These yarns are long wearing and stable. Use them for knitting items that get hard use, such as socks.

Think of the color possibilities. While a good, structurally sound cable can be spun from any one color of fiber, that could get rather boring fast and there are all those singles to play with. Try a cabled yarn with two colors; with 3 colors, with 4 colors and things get very interesting very fast.
Also, there is no “rule” that all the plies going into the cable must be the same fiber or even the same size or that there must be 4 singles in the cable. Try 2 singles plied together S and then add a thin binding thread as the final Z plying step. Try cabling singles of a luster long wool breed with silk. Try using a commercially spun yarn in your cable. The possibilities are endless.