Collar

Besides the common definition, a BDSM symbol of attachment of a submissive to a Dominant through collar made of metal, leather, or other material. Similar to a wedding band in vanilla society. Most collars are equipped with rings where a leash or restrain can be attached, like a dog's collar.

Wearing his collar to sleep was uncomfortable, but it reminded Arthur all of the time that his Master loved and owned him, and he felt safe.