How an NFC card works.

NFC stands for near field communication. It is the same technology you use to pay with your phone or a bank card at the till: two devices exchange a little data when they are almost touching. On my cards it does one simple job. It hands the phone a link.

What happens when you hold a phone to it.

Inside the card is a tiny chip with a thin antenna around it. The chip has no battery. When a phone comes close, the phone's antenna creates a weak field that wakes the chip for a moment, and in that moment the chip sends whatever is written on it. The phone sees that it is a link and opens it, just as if you had tapped a link in a message.

The guest never notices that whole conversation. In a konoba they don't have to search or type anything: they hold the phone to the card on the table and they are already looking at the menu or the review window. A passenger in a taxi holds the phone to the back of the seat and has the driver's number.

The phone has to come almost close enough to touch. From further away NFC doesn't work, and that is deliberate: nobody can read the card in your pocket from across the room.

Why no app is needed.

Reading NFC is built into the phone itself. Once it has the link, the phone opens it in the browser it already has, so nobody has to install anything, visit an app store or create an account. For someone who has just got into a taxi or reached an apartment late at night, that is the difference between “I'll give it a go” and “not now”.

The card is completely passive. It can't read anything from the phone, it doesn't know who touched it, and it sends nothing except the link written on it. There is nothing to break, either: no battery to run down and no software to update.

Which phones read the card.

• iPhone. Every model from the XS onwards reads the card on its own while the screen is on. A notification appears at the top of the screen, and you tap it. Hold the phone by its top edge, the one near the camera.
• Android. Most phones have NFC, but on some it is switched off in the settings. When it is on, the link usually opens straight away. Hold the middle of the back of the phone to the card.
• Older and cheaper models. Some have no NFC, or won't read it without an extra step. For them, the card carries a QR code.

That is why none of my cards relies on the chip alone. Every one also carries a QR code, so the camera leads to the same place, and a contact card also has a printed address you can type. More on that in tap, scan or type.

What is written on the chip.

The chip holds very little, just enough for one address. I don't write your details on it, I write a link. The link goes wherever the card should go: to your digital profile with your number and a save-contact button, to Google's review window, to a menu or to a guest guide.

Same chip, different link, different job. A contact card, a review card and a guide card are identical inside. The only differences are the address on the chip and what is printed on the card.

A card can be re-pointed.

I write my own short address on the chip, and it forwards the visitor to the real destination at once. When the destination changes, say you get a website instead of a profile or open a new Google profile, I only change where the short address leads. The card stays the same, in your wallet or on the table, and doesn't have to be made again.

That same short address is what lets you see how many times a card was tapped or scanned, with no personal data at all. I explain how in the article on counting taps and scans.

The chip doesn't have to be in a card.

The same kind of chip can sit in a table stand, a plaque on a door or wall, a keyring or a sticker you put wherever you need it. I make the stand and the plaque by 3D printing, in the style of the place they stand in. A taxi can have a chip on the back of the seat, an apartment by the front door and a konoba on every table.

Chips don't like metal. If a card, sticker or plaque has to sit directly on a metal surface, tell me first, because there an ordinary chip reads poorly or not at all.

In short, an NFC card is a printed piece with a link inside it. It is worth as much as the page it opens. That is why, for me, the card is never the whole job: what opens when someone taps it matters just as much.