When to hold a girl by the waist, when to hold them by the shoulder?

When to hold a girl by the waist, when to hold them by the shoulder? Is there an unwritten rule concerning this when taking pictures or walking side by side or a small hug, etc.

(One of my friends pointed it out that I've always hold girls by the side of their shoulder and never the waist)
When to hold a girl by the waist, when to hold them by the shoulder?
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