What to know:

• A passport card is a wallet-sized federal ID that can be used for domestic air travel and meets REAL ID requirements.

• If you already have a valid passport book, you can get a passport card for $30 – and you don’t have to wait until your passport expires.

• A passport card can be used for some international travel by land and sea, but it cannot be used for international flights.

I was today’s year old when I found out I could get a second passport that fits in my wallet.

That, ladies and gentlemen, is what my former journalism professors would call a solid lede. It’s relatable, it introduced you to something new, and, hopefully, it created just enough curiosity to keep you reading.

And now that I have your attention, let’s explain.

The second passport I’m talking about is called a passport card, and it’s exactly what it sounds like: much of what you get from a traditional passport book, packed into a card about the size of your driver’s license.

The U.S. State Department has been issuing passport cards since 2008, but they’ve gained a lot of attention lately because of one simple thing: they are TSA REAL ID-compliant.

Right about now, you’re probably thinking, “Why do I need to carry around my passport book when I can use one of these things instead?”

Good question – let’s break it down:

• It can’t completely replace your passport: This is the biggest catch. A passport card cannot be used for international air travel. If you’re flying to Europe, Asia, South America, or even just hopping on a plane to Canada or Mexico, you’ll still need your passport book.

The card is limited to entering the United States at land border crossings and sea ports of entry from certain countries (more on that below). One other thing: because it’s a plastic card instead of a booklet, there are no visa pages.

• It’s great for crossing the border and some cruises: The passport card can be used to enter the United States by land from Canada and Mexico, and by sea from Canada, Mexico, Bermuda, and Caribbean countries.

That makes it particularly useful for people who frequently cross the Canadian or Mexican border – and potentially useful for cruise passengers. Just remember the rule above: if you unexpectedly need to fly home from another country, the card won’t get you on an international flight back to the United States.

• You can save a trip to the DMV: Not every driver’s license is a REAL ID. Several states, including New York, New Jersey, Pennsylvania, Michigan, California, and Washington, continue to issue standard driver’s licenses that aren’t REAL ID-compliant. If you have one, you don’t necessarily have to replace it just to get through airport security.

Since the passport card is REAL ID-compliant, TSA accepts it as identification at airport security checkpoints for domestic flights. And unlike your passport book, it’s small enough to keep in your wallet.

• It’s not free, but it’s not terribly expensive…maybe: Here’s where the math gets a little weird. How much a passport card costs depends on whether you already have a passport.

A first-time adult applicant pays $30 for the card plus a $35 acceptance fee, for a total of $65. But if you already have a passport book and are eligible to use the renewal process, you can apply for your first passport card for just $30 – and you don’t have to wait until your passport book expires. If you’re renewing your passport book anyway, adding the card also costs $30.

• It lasts longer than a Florida driver’s license: Passport cards issued to adults 16 and older are generally valid for 10 years. Florida driver’s licenses and ID cards are generally valid for eight. For someone who already has a passport book and can get the card for $30, that works out to about $3 a year for a wallet-sized federal ID.

• It proves you are a U.S. citizen: Your driver’s license may prove who you are, but it doesn’t prove you’re a U.S. citizen. A passport card does both.

The federal government recognizes both passport books and passport cards as evidence of U.S. citizenship, giving the card another potential use when you need to establish both citizenship and identity without carrying around your passport book.

• It’s great for CBP’s Ready Lanes: If you regularly drive across the border, there’s another advantage to having a passport card: it contains technology that allows use in U.S. Customs and Border Protection’s Ready Lanes at land ports of entry.

Here’s how it works: as you approach the inspection booth, hold the card up so its information can be read and retrieved before you reach the CBP officer. This can help speed up the inspection process – and it’s something an ordinary passport book isn’t designed to do.

• It’s a pretty good backup ID: Losing your driver’s license is inconvenient. Losing your passport book can be a much bigger headache. A passport card gives you a second government-issued photo ID that’s small enough to keep in your wallet, while allowing you to leave your passport book safely at home.

It’s accepted by TSA for domestic flights and can come in handy whenever you need another form of federal identification. For someone who already has a passport book, $30 buys a backup ID that’s good for 10 years.

When you might not need a passport card

Of course, for every rule, there are exceptions. And in this case, it’s not so much an exception to the rule as it is overlapping benefits that make passport cards unnecessary for some U.S. citizens.

If you happen to live in any of these five states – Michigan, Minnesota, New York, Vermont, and Washington – you may already have another option because those states offer something called an Enhanced Driver’s License.

An Enhanced Driver’s License, or EDL, goes beyond an ordinary REAL ID driver’s license because it establishes both identity and U.S. citizenship. Just like a passport card, an EDL can be used to re-enter the United States at certain land and sea border crossings without a passport book. EDLs are particularly useful for people who frequently travel between the United States and Canada. But, just like a passport card, an Enhanced Driver’s License cannot replace a passport book for international air travel.

At this point, some of you may be asking about a Global Entry Card and how a passport card stacks up against it.

Global Entry members may already have some of these benefits, but not all of them. If you’re enrolled in Global Entry, take another look at the physical card that came with your membership. TSA accepts Global Entry cards as identification for domestic flights, and the card can also be used for certain land and sea border crossings. Global Entry cards can also be used in SENTRI and NEXUS expedited lanes when entering the United States, although additional rules apply.

But a Global Entry card isn’t a substitute for a passport book for international air travel, and unlike a passport card, it does not by itself establish that the holder is a U.S. citizen. So, if you already carry a Global Entry card, a passport card may offer fewer additional benefits – although the passport card still has value as proof of both identity and U.S. citizenship.

So, do you need one?

Hard to say.

For most Americans, a passport card is a convenience, not a necessity. If you already have a REAL ID-compliant driver’s license and a passport book, a passport card doesn’t give you many new capabilities. What it does is combine several of them into one wallet-sized document.

But convenience is kind of the whole point.

For $30, someone who already has a passport book can get a wallet-sized federal ID that is good for 10 years, proves U.S. citizenship, works at TSA checkpoints, and can even be used instead of a passport book for certain trips by land or sea. More importantly, it means you can leave your passport book somewhere safe instead of carrying it around when you don’t need to.

Is that worth $3 a year? That’s a question only you can answer. As for me, I was today’s year old when I learned passport cards existed – now I’m $30 poorer.