Size 6, 4.00 mm, Before the 20th century fine knitting was done on steel knitting needles but bone was the material of choice for coarse knitting.
Bone Knitting Needles
Knitting Needles 101 - Magda Makes
7 Double Point Clover Bamboo Knitting Needles, Set of 5 is 60% lighter than aluminum, they have a tempered, round-shaped point that never splits
Clover 7 Bamboo Size 10 Double Point Knitting Needle Set
Brittany Single Point Knitting Needles, 10 inches long, Birch
Knitter's Pride Zing Needles are made from superior grade light weight aluminum that is warm to the touch and gentle on the hands. Perfect points are flawlessly tapered. A smooth joint ensures easy stitch glide and no snagging. Flexible cords lay flat with no kink or twist. Each size features it's own radiant color for easy identification. 6 inch Double Pointed Set Socks- A set specially designed for small projects; ideal for sock knitters.
Socks Kit - Zing Double Pointed Needles Set - Knitter's Pride
What can I knit with 6mm needles? - Quora
Size 6 10 Double Pointed Bone Knitting Needles - Wm. Booth, Draper
Boye Doublepoint Knitting Needles Set of 5-US 6/4.25mm
40pcs/Set Knitting Needles Kit Carbonized Bamboo Double Pointed
Set of 4 antique knitting needles. Made of bone. France Size 2.25 cm, Size 13 UK ,Size 1 US although on the paper label it is written 2.5 cm / 0.98
4 French Antique Double Pointed Bone Knitting Needles 2.25 cm Size 13 UK Size 1 US 19.5 cm Long, Retro Old Knitters Equipment from France
Knitting needle - Wikipedia
Double Point Knitting Needles
Knitting Needles 101 - Magda Makes