GB / T 1235- -76 (old national standard)
Standard is outer diameter * wire diameter
Wire diameter 1mm, 1.5mm, 1.9mm
2.4mm, 3.5mm, 4mm, 5.7
mm, 8.6mm all implement this standard.
GB / T 3452.1-2005 (new national standard)
Standard is inner diameter * wire diameter
Wire diameter 1.8mm, 2.65mm, 3.55
mm, 5.3mm, 7mm all perform this
standard.