Neck and limb rings are used to make the head and limb easier on the body.
The diameter of the neck is measured from the opposite point.
Referencing the image, that would be the lower part.
Ringgröße mm | Durchmesser A mm | Durchmesser d mm |  |
38 | 38 | 31 |
40 | 44 | 36 |
50 | 50 | 40 |
55 | 53 | 45 |
58 | 56 | 48 |
60 | 59 | 51 |
70 | 69 | 60 |
74 | 74 | 64 |
78 | 78 | 68 |