The invitation is lovely and it's addressed to one person: you. No "and guest." And you have someone, a partner of eight months, someone new but real, or you're facing a destination wedding full of strangers and want your person beside you. Somewhere between the envelope and your phone lives a question with a surprisingly high failure rate: can I ask?
First, decode the envelope, because the missing plus-one is almost never a message about you or your relationship. Guest lists are built under a per-head number, and plus-ones are the first category squeezed, usually by a blanket rule the couple applied to stay sane: married and engaged partners only, or partners we've met, or couples together over a year. Rules like that exist precisely so the couple doesn't have to adjudicate each relationship, and your solo invitation most likely means you fell on the far side of a line drawn for everyone, not that anyone evaluated your person and voted no.
That decoding matters because it determines the register of the ask. You are not correcting an oversight or appealing an injustice. You're asking whether an exception is possible to a rule that exists for reasons you can't see, which means the ask must be genuinely, structurally easy to refuse. That's the entire craft of it: a plus-one request the couple can decline in one comfortable sentence is a normal question; one they can't decline without feeling like villains is pressure, and pressure sent to people drowning in wedding logistics is how guests become anecdotes told for years.
The clean version, sent well before the RSVP deadline, to whichever half of the couple you're closest to:
Quick question, and I want to be clear the answer can absolutely be no: is there any room for me to bring Dana? Completely understand if the numbers are locked or there's a partners rule, I know how brutal guest math is, and I'm coming and celebrating hard either way. Just figured I'd ask rather than wonder.
Look at the load-bearing parts. The permission to refuse comes first, before the ask, so they read the question already holding the exit. The guess at their reason ("numbers," "partners rule") does something subtle: it shows you understand a system exists, so declining requires no explanation, they can just say "yeah, we had to draw a hard line, sorry!" And "I'm coming either way" removes the hostage scenario entirely; the request that even faintly implies attendance depends on the answer is the version that ends up in a forum post with two thousand comments.
Now the rules around the message, which matter as much as its wording. Ask once. The answer, whatever it is, is final, and any follow-up, negotiation, or visible sulk converts your reasonable question into the story. Never RSVP for two uninvited, writing a guest onto the card is the single most reviled move in the genre, and never just bring someone, which is its armed version. Don't route the ask through the maid of honor or a parent; sideways requests read as campaigns. And absorb a no like it costs you nothing, "totally get it, see you on the dance floor," because it mostly doesn't: you'll be at a wedding, which runs on shared tables and open bars, the most solo-friendly party format humans have built.
And one honest gate before you send anything: check whether your case is strong enough to spend the ask on. A live-in partner the couple somehow doesn't know about, a destination wedding where you'd know no one, a long relationship that predates their rule's cutoff, those are reasonable exceptions to inquire about. A three-week situationship is not, and asking spends real goodwill on a weak hand. If you're on the fence about whether your case clears the bar, it probably doesn't, and the strongest move available is the one nobody regrets: go solo, be great at their wedding, and tell your person about it after.
One well-built ask, one graceful landing either way. That's the entire etiquette, and it fits in four sentences.