The "marriage cross" or "wedding cross", a minor Christian symbol which signifies Christian marriage.  For discussion, see the book Outward Signs: The Language of Christian Symbolism by Edward N. West (1989), etc.  The same basic idea can receive a variety of different artistic treatments (for one version, see illustration 320 on page 267 of The Bride's Book: A Pictorial History of American Bridal Dress by Catherine S. Zimmerman, 1985, ISBN 0-87795-704-5).
Own work - Self-made graphic, declared to be PD.
AnonMoos