Nipple Piercing Facts and FAQs - How Do I Know if My Nipple Ring Is Infected?
Male Nipple Piercing | Elayne Angel's Piercing Bible
How to Care for a Nipple Piercing
I have a great example of how wonky flat or inverted nipples can look pierced. Left is when they are erect, the bar sits perfect. Right is relaxed, looks way too deep :