In recent years the rules have relaxed about what's considered appropriate wedding attire - while it used to be cocktail dresses and suits only, these days almost any nice outfit goes.
However one rule prevails, whether you're a friend or family member of the bride and groom - ladies, do not wear white to another woman's wedding.
But one mother-in-law obviously threw that rule to the wind with her chosen outfit for her son's wedding - a floor, length, silky white dress.
A picture of the wedding party was shared to a wedding-shaming forum on Reddit, with faces covered to protect their identities.
"Groom is the red dot. Can you guess who is the bride and who is the psycho mother in law? Who does this!?!" the poster captioned.
Both women are flanking the groom, and if it wasn't for the bouquet of flowers we presume the bride is holding, it would be tough to tell the two apart.
One commenter had a succinct response to the question, writing it was "easy" to tell who the groom's mother was.
"The wrong woman in these cases is always literally leaning against the groom," they pointed out.
"Body language, a silent gift," another agreed.
Others questioned the mother-of-the-groom's reasoning.
"I don't understand why you would want to look like you are marrying your child," one person wrote.
"Jeez, not only is that a terrible dress choice, the bride is actually further away from the groom than his mother," another pointed out.
"Gross. You can guarantee she's gonna crop this so it's just her and her son," another agreed.
It's not the first terrible wedding outfit choice to go viral.
Last year a wedding guest was accused of "upstaging" the bride after donning a sheer, red lace dress to a wedding reception.