Note: In this article, the number of plies refers to the number of
strands of fiber within the yarn. Confusingly, a ply number is used as a
yarn weight in some countries, e.g. a fingering weight yarn is often called
"4-ply" in the UK and Australia, regardless of how many strands the yarn actually has.
Image © LoveKnitting
As the name suggests, this is just one strand of fiber. With a single ply
yarn, manufacturers walk a tightrope between too much twist - leading to an
'unbalanced' yarn that might leave you with a biased piece of knitting -
and too little twist, giving a yarn that breaks and pills easily.
The rounded cross-section of a single ply yarn allows individual stitches
to fill all the space available to them, giving a cohesive look and a cosy
feel to the fabric. The stitches are smooth and bright, without the shadows
that come from multiple plies.
Singles are not well-suited to items which will get a lot of wear and
tear, like socks or gloves for example.
Two-ply yarns have a definite wavy edge, as the two strands wind around
each other with nothing else to fill out the gaps. They are stronger than
single ply yarns, with the fibers more supported and protected from
breaking by the extra twist. They are also more balanced and unlikely to
produce a biased fabric, with the excess twist from the individual plies
being used to twist each around the other.
The wavy edge means they can be good for color-work, blending the borders
of color changes. They are also traditionally used for lace knitting.
Three-ply yarns are more durable than single or two-ply yarns, with all of
the twist in the individual plies plus the extra twist to ply them
together. They are more round in cross-section than two-ply yarns, less
round than single ply yarns, but stronger.
The more plies in a yarn, the stronger, more durable and more rounded it
becomes, giving good structure to textured stitches and cables. The more
plies you add, the more dense the yarn becomes, as all available space
within the column of yarn is used up.
Image © LoveKnitting
Plied yarns can themselves be plied together. This is a great construction
for a wool fiber like Merino, with the relatively short, soft but delicate
fibers being protected by the high total twist in the yarn. These yarns are
perfect for textured stitch patterns like seed stitch (UK = moss stitch)
and cables. They are strong, and resistant to pilling. But all those plies
also keep the fiber very much under control, so it's not a yarn to choose
if you're after a more rustic look.
Image © LoveKnitting
This is where the plies themselves are knitted, or chained together to
form the yarn, a bit like i-cord. Using a chained construction for fibers
that have no inherent elasticity, like silk or plant fibers, gives the yarn
as a whole some elasticity. This works because a knitted fabric can stretch
in all directions and can produce a more forgiving yarn for these smooth,
S-on-S plied yarns
Image © YarnSub
S-on-S plied yarns—also called multi-thread or millefili yarns—are
constructed from ultra fine single plies that are S-twisted into 2 ply
yarns that are themselves plied together with an S-twist, hence the name S-on-S.
This differs from traditional yarns where most often a single ply is spun
with a Z-twist, and then two or more of these are plied together in the
opposite direction, giving a yarn with a final S-twist.
The S-on-S construction gives flexibility to the big mills, which can use
the fine yarns for commercial knitwear, while also having them as building
blocks for yarns suitable for use by the handknitting industry.
The result of the S-on-S construction is a light, airy yarn with lots of
bounce. But it's also sleek and well-behaved, with the high level of twist
keeping the fibers firmly caught up within the body of the yarn. Since the
fibers can't escape easily, the finished fabric is not prone to pilling.
They're usually made with superwash wool, treated to smooth the individual
fibers (and stop the naturally-present scales from locking together and
felting). That also means that the finished fabric is liable to relax and
stretch with only a little encouragement, i.e. from washing, blocking or
Yarn twist direction, along with your method of knitting or crocheting, affects whether the yarn splits as you work.
Blown yarns are made by blowing fibers into a mesh tube. This gives them interesting properties compared with plied yarns.