Yarn ply and construction
A ply is one strand twisted with others. Regional 4-ply or 8-ply weight names do not guarantee that exact strand count.
Count strands, compare fabric
Untwist a short cut sample to see its component strands. Single-ply, chainette, blown and tape yarns can share a weight category while having different surfaces.
Regional names
UK 4-ply commonly names a fingering-weight yarn; Australian 8-ply commonly names DK. Compare the metric label, length per mass and washed gauge instead of using ply count as a universal thickness conversion.