Why a seam stitch?
When I knit a guernsey (gansey) I always incorporate seam stitches. These are columns of purl or garter stitches, running down each side of the garment as if they were a seam. Apart from being a tradition it has more than a couple of practical uses.
If you use garter stitch for the 'seam' then each purl 'bump' signifies 2 rows and that makes counting rows much easier; anyone who has tried to count rows up stocking stitch knows how easy it is to lose your place as your eye loses concentration. With garter stitch then it is easy to keep your place with a thumb nail when you blink.
As you reach the beginning of the gusset, the start of the patterns or the sleeve position, the artificial seam line saves lots of counting stitches again to find the exact dividing line of the front and back panels;.the 'seam' then divides and splays outwards to follow the widening gusset edges, only to
come together when the gusset is narrowed back to the original 'seam'
partway down the sleeve.
Sometimes the 'seam' is more decorative than just a single purl stitch column. I have seen pretty little textured patterns, over 2 or 3 stitches, these also help to make the plain stocking stitch area a little more interesting and hide any looseness at the changeover of the needles. The seam also presents a useful position for starting a new ball of yarn.
I have been reading very old knitting primers (for schools to teach knitting), dating from the mid-19th century, and the instructions (recipes!) for knitting stockings invariably include the same artificial seam. This seam may hark back to the construction of stockings from fabrics which were in common use prior to the reign of Queen Elizabeth 1 in England. Research seems to agree that Her Majesty received a present from one of her household of the new fashion from the continent of hand-knitted silk stockings, which she found so comfortable that she did not want to wear her 'old' hand-sewn' fabric stockings again.
Incidentally, sewn fabric stockings continued to be worn in Scotland for another couple of hundred years, and the diagonal pattern of Argyle stockings is thought to reflect the style of fabric stockings made of tartan fabrics with their bold checkered patterning, which being cut on the bias showed the checkered pattern in a diagonal form.
The pretend-seam in knitted stockings, in the mid-19th century primers emphasised the practical purpose of the vertical line of contrasting stitches. If the 'seam' is knitted in alternate plain and purl rows, then it is easy to count rows, each 'purl-bump' representing 2 rows. This helps also in counting rows for shapings and anyone who has knitted a cabled stocking knows how infuriating it is to be accurate in counting 4 or 6 rows between twists. If you had a garter-stitch seam you would know that every other purl-bump (or every third) indicated your row for crossing your cables. The seam stitches were continued down the heel until the need for counting rows stopped as you turned the heel. The 'seam' also gave a position for shapings which were necessary on long or knee-length knitted stockings such as I wore when I was a young schoolgirl.
Ladies familiar with fashion stockings will know that some will incorporate a back seam, which may be purely decorative. However in the mid-20th century and earlier, these seams really were to join cut fabric into a 'tube'. Incidentally, in those days stockings were so expensive that it was worth paying to have them repaired if you snagged them.
This is a fragment from an 1883 school knitting manual by E Lewis, for a womans stocking to be knitted on size 16 needles (1.50mm):
But this instruction continued in knitting recipes until the late 1930s. In 1939 Mary Thomas, widely known as a modern knitting guru, also recommended incorporating a seam stitch in stockings! Here is a scan of her 'stockings' page from 'Mary Thomas's Knitting Book' (apologies for fuzziness at the left edge - her book is very thick and I did not want to damage it.)
Does anyone nowadays add a seam in their stockings? If you want to knit long stockings on circular needles, a seam would avoid having to use a stitch holder for the centre back stitch shapings, and help to align the heel with the shapings.