Promise rings for men are not extremely common, so they don’t adhere to any hard rules as to style.  Because of this, there are a large variety of ring shapes and styles available. Traditionally, promise rings denote the commitment that a couple has made to each other.  The promise ring will generally be worn before an engagement when the couple is committed to one another, but not ready to announce an engagement.

Above text was excerpted from: http://promiseringsformen.org