The new do's and don'ts for wedding guests.

You don’t want to be THAT guest who ruins your friend’s wedding.

Soon-to-be brides certainly don’t need to be told that the 21st century has ushered in a new era of wedding etiquette. They’ve likely already considered sending eco-conscious invites via email — and appeasing their hashtag-happy generation with a custom, Instagrammable wedding slogan.

But what about the guests?

If you have “wedding” in your social calendar, you’re going to want to put your best foot forward. And that means staying up-to-date on what’s been crossed out of the rule book, what’s been tweaked, and what’s, well, really not up for debate.

Diane Gottsman, an etiquette expert and founder of The Protocol School of Texas tells us all about modern wedding guest etiquette.

1. The old rule: Thou shalt not steal the bride's thunder by wearing white to a wedding — oh, and black's out of the question, too.

"Black used to be taboo, yes," Gottsman said. "In most cases, if this is the second marriage for the bride, those sorts of rules are already thrown out the window. And in other cases, I believe that as long as you don't show up as if you're going to a funeral, you're totally fine."

Tradition aside, a "shroud-looking garment" just might not be the best choice aesthetically. You can liven up the all-black look with some killer shoes and playful jewelry.

As for white?

"Traditionally, you would not wear white. White and ivory should be left for the bride — and that still holds true today," said Gottsman. "Of course, you can wear a dress with some white in it, or have white somewhere within your outfit, but you shouldn't plan to show up wearing an all-white ensemble."

Read more: Did this friend commit the ultimate wedding no-no?

2. The old rule: Thou shalt not take photos on your smartphone, because ... wait, what's a smartphone?

"Now that everyone's phone takes photos, and now that those photos are promptly uploaded to social media platforms, you really need to make sure that you're not trumping the bride and groom," said Gottsman.

Some will encourage you to photograph the event, and even give you a hashtag to use on all the wedding photos. But if you aren't sure what they want, err on the side of caution. Which is to say, don't post anything.

"Often, a guest will post before the bride has even had a chance to post herself, and that can be upsetting. You just don't know how they're feeling, and with tensions running high already, it's best to put down the camera."

Secondly, if there's a professional photographer, don't get in their way. The bride and groom are likely looking forward to having photos taken by the professional they paid to do the job. They'll appreciate it if you allow the photographer his or her space.

"Oh, and you also do not want to tag the bride in a photo that's unflattering," concluded Gottsman. "And, of course, you never want to post pictures of children without permission."

3. The old rule: Thou shalt send in your RSVP via snail mail.

"Sure, things have changed, and some people really are conscientious of going green," Gottsman said. "But it doesn't mean you can choose your own response method or get lazy with it."

The best way to ensure you're doing the right thing is to answer the invitation in the form in which it's requested. So, if the couple emails you and provides an RSVP email, respond with a courteous note to that email. Likewise, if they invite you with beautiful stationery, don't plan on texting them with a, "Yup, I'll be there."

Not only does that take away from the exciting, formal tone they've set, but it also makes their life difficult. Someone's collecting all those little RSVP cards, so help them out by keeping all the responses in one place.

And remember — simply telling someone you're coming does not constitute an RSVP.

"If you're invited to a wedding, and you run into the bride at the grocery store and say, 'Hey, yes, we'll be there,' that just doesn't cut it," Gottsman laughed.

Read more: The 15 most appalling stories of wedding guests behaving badly.

4. The old rule: The bride and groom are expected to create a registry. And thou shalt not stray from the items on the list.

"The truth is, a registry is still pretty fabulous because it's specific and provides details on exactly what the bride and groom are going to need," Gottsman said. "They still serve a purpose. So, if you're provided with a link to a registry, you should still go by it."

If you plan to go wild and get the happy couple something they didn't ask for, Gottsman advised that you should include a gift receipt. She also conceded that gift cards and monetary gifts are fine, especially if you feel that the registry is a bit out of your budget.

But, in the end, this rule isn't going anywhere. In fact, there's even more of a reason to abide by the registry system in 2015.

"Nowadays, people are getting married a little older," Gottsman added. "They may already be living together, and they might have their toaster and their blender. You can't be sure your gift will actually be helpful to them unless you check that list."

5. The old rule: Thou shalt respectfully participate in all wedding traditions, including the throwing of the bridal bouquet.

"Some people love going out and catching the bouquet. Other girls may feel like, 'That's so outdated. I don't want to put myself in that position where I'm vying for the flowers,'" Gottsman said.

"But whatever you believe, this day is not about you, and if the bride has chosen to include a tradition such as this one in her big day, that's her prerogative."

Rather than making a scene, simply excuse yourself quietly.

"Don't just stand off to the side, because people may chide you in a friendly way to get out there and participate, and you'll have to respond," Gottsman added. "You do not want to appear adversarial, at least at this particular moment in time, because this moment is not yours."

Read more: “Don’t judge me, but… I won’t be giving my friend a wedding present.”

6. The old rule: Thou shalt attend every second of the wedding ceremony, which will most likely be in a religious setting. Then, you can attend the party.

"Some people will think, 'Oh, I don't want to sit through that long religious ceremony. I just really want to go to the fun part,'" said Gottsman. "Well, if the couple has invited you to the wedding, they're expecting you to join them for the full experience."

A ceremony and reception go hand in hand, meaning this rule is here to stay.

"Trust me, the bride and groom will remember who shows up and who doesn't. And they'll remember who leaves early, too."

It's true that the tradition of the ceremony has changed enormously over the years. Many are now taken outside of churches or synagogues and instead held on beaches or balconies or hot air balloons. But whatever that ceremony consists of, you're sharing an important moment with the couple. Be there for them.

7. The old rule: Thou shalt consider bringing a plus one if and only if the invitation makes it clear that a person other than you is being invited.

"The invitations may look different, but the tradition is the same," said Gottsman. "If your boyfriend or girlfriend's name is not on the invitation, or if it doesn't include a crystal-clear 'plus one' addendum, they're not invited."

That may be uncomfortable for some invitees who are left to tell their significant other they didn't make the cut. But try not to bug the couple about it. The exception is if you're engaged or married. In that case, the invite blunder was probably just an oversight on the part of the bride or planner.

Have we missed any basic etiquette that guests constantly mess up?

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