QR code glossary

Every term used on this site, in one place. Each entry is a sentence or two; where there is more to say, it links to the article that says it. Other pages link to entries here on the first use of a word, so you can arrive at any letter from anywhere.

A

Alignment pattern

A small square, 5 modules a side with a dark centre, placed inside larger codes so a scanner can correct for a curved or tilted surface. Version 1 has none; version 40 has 46. See how QR codes work.

Alphanumeric mode

One of the four encoding modes. It covers digits, capital letters, space and eight symbols, and stores two characters in 11 bits. Lowercase letters are not included, which is why they force byte mode. See encoding explained.

App Clip code

Apple’s circular code that opens a lightweight part of an iOS app without installing it. Readable by iPhone camera or NFC, not by Android.

App Link

Android’s verified deep link. A normal https address that Android opens in the app that has proved it owns the domain, or in the browser if the app is not installed.

Aztec

A 2D barcode with a bullseye finder in the centre and no quiet zone. Common on airline and rail tickets. See QR codes vs barcodes.

B

Barcode

Any machine-readable pattern that encodes data. Stripes on groceries are 1D barcodes; QR, Data Matrix and Aztec are 2D barcodes.

Block

A group of data codewords with its own set of error-correction codewords. Small codes have one block; version 40 at level H has 81. Blocks are interleaved on the grid so damage spreads across them.

Byte mode

The encoding mode that stores any byte in 8 bits. Used for web addresses, since they contain lowercase letters, and for any text outside the alphanumeric set.

C

Character count indicator

A field of 8 to 16 bits after the mode indicator saying how many characters follow. Its length depends on the mode and the version.

Codeword

A group of 8 bits, the unit that error correction works on. A version 1 code has 26 codewords. One wrong module spoils its whole codeword.

Contrast

The difference in brightness between dark and light modules. Scanners need dark modules to be clearly darker than light ones; a dark code on a light background is the safe choice.

D

Dark module

A single module that is always dark, beside the bottom-left finder pattern. It is a fixed reference point and carries no data.

Data codeword

A codeword holding your message rather than repair data. Version 1 at level M has 16 data codewords and 10 error-correction codewords.

Data Matrix

A 2D barcode with a solid L-shaped border along two edges. Used on electronics, medicine packs and very small labels, from 10 modules a side.

Deferred deep linking

A deep link that survives an app install: the user scans, is sent to the app store, installs, and the app still opens the right screen on first launch. Needs a linking service to remember the intent.

Deep link

A web address that opens a specific screen inside an app rather than a web page, such as a product in a shopping app. Needs the app installed and set up to claim the link.

Denso Wave

The Japanese company, part of the Toyota group, whose engineer Masahiro Hara led the design of the QR code in 1994. It holds the patent and has said since 1999 that it will not enforce it.

Dynamic QR code

A code holding a short redirect link. A service resolves the link to the real destination, so the owner can change the destination and count scans. Works only while the service does. See static vs dynamic.

E

ECI

Extended Channel Interpretation. A header inside the data that says which character set the bytes use. ECI 26 means UTF-8. Most phones assume UTF-8 whether or not the header is present.

EMVCo

The body run by the card networks that publishes payment standards. Its QR specification defines the merchant-presented and consumer-presented payment codes used in many countries.

Error correction

Extra codewords computed from the data so a scanner can repair modules it misreads. It is why a scratched code or one with a logo still opens. See error correction explained.

Error-correction codeword

A codeword computed from the data codewords by Reed-Solomon coding. With r of them in a block, up to r/2 codewords in that block can be wrong.

Error-correction level

One of L, M, Q or H, reserving about 7, 15, 25 or 30% of the codewords for repair. M is the default; H is for codes with a logo.

F

Finder pattern

One of the three large squares in the corners, each a dark 7 by 7 square with a light ring and a dark 3 by 3 centre. Their 1:1:3:1:1 ratio lets a scanner find the code from any angle.

FNC1

A flag at the start of the data saying the content follows GS1 rules for product and batch data. Retail and pharmacy scanners parse it; phones show the raw text.

Format information

Fifteen bits, stored twice beside the finder patterns, giving the error-correction level and the mask. The scanner must read these before it can read anything else.

Frame QR

A Denso Wave design with a canvas area in the middle for a picture that costs no error correction. Needs a compatible reader. See variants.

G

GF(256)

The Galois field of 256 elements, the arithmetic Reed-Solomon uses. Elements are bytes, addition is XOR, and multiplication is reduced by a fixed polynomial. See masking and Reed-Solomon.

GS1 Digital Link

A GS1 format that puts a product’s identifiers, such as its GTIN and batch, into an ordinary web address, so one QR code can serve a till, a phone and a recycling scheme.

H

Hash

A fixed-length fingerprint computed from data by a one-way function such as SHA-256. Redirect services hash a scanner’s network address to count unique visitors without storing the address. See analytics explained.

I

Interleaving

Placing codewords from different blocks alternately on the grid, so that a scratch damages a little of every block rather than a lot of one.

Inverted code

Light modules on a dark background. Many scanners read it, some do not, and the finder patterns still need a quiet zone of the background colour.

iQR

A Denso Wave code, square or rectangular, up to 422 modules a side and about 40,000 digits, not an ISO standard. Read by industrial scanners only.

ISO/IEC 18004

The international standard that defines the QR code. First published in 2000; the fourth edition is dated August 2024.

J

JIS X 0510

The Japanese Industrial Standard for the QR code, published in January 1999, which preceded the ISO standard.

K

Kanji mode

The encoding mode for Japanese characters in Shift JIS, storing each in 13 bits instead of the 16 that byte mode would need.

L

Logo

A picture placed over the middle of a code. It works by spending error-correction budget, so it needs level H and should stay under about 20% of the area and away from the finders.

M

Mask

One of eight fixed patterns XORed over the data area so the code has no large blank regions or accidental finder patterns. The scanner reads which mask was used from the format information and undoes it.

MaxiCode

A fixed-size 2D barcode of hexagon dots around a bullseye, holding about 93 characters. Used on UPS shipping labels.

MeCard

A compact contact-card format, MECARD:N:Surname,Given;TEL:...;;, smaller than a vCard. Android handles it well; iOS is less consistent.

Micro QR

A small QR variant with one finder pattern, 11 to 17 modules a side, holding at most 35 digits or 15 bytes. Thin phone support.

Mode indicator

The 4 bits at the start of each data segment saying which encoding follows: 0001 numeric, 0010 alphanumeric, 0100 byte, 1000 kanji.

Model 1

The original 1994 QR code, up to version 14 with no alignment patterns. Obsolete.

Model 2

The 1997 revision, with alignment patterns and 40 versions, that ISO/IEC 18004 defines and every phone reads. When this site says QR code, it means Model 2.

Module

One of the small squares in the grid. Dark is 1, light is 0. The module is the unit of size: version 1 is 21 modules a side.

N

NaviLens

The maker of the Accessible QR Code, a high-contrast coloured marker wrapping an ordinary QR code, readable by the NaviLens app from up to 12 times the usual distance and spoken aloud for people with low vision.

NFC

Near Field Communication, the tap-to-read radio in phones and payment cards. Often paired with a QR code on the same sign, since the tag costs money and the code costs nothing.

Numeric mode

The encoding mode for digits only, storing three digits in 10 bits, about 3.3 bits a digit. A phone number in numeric mode is less than half the size of the same digits in byte mode.

P

Pad codeword

Filler added after the data to fill the version’s data capacity, alternating between 11101100 and 00010001 so the code never has a large blank stretch.

Payload

The text stored in a code: a web address, a WIFI: string, a vCard, a phone number. The phone reads the payload and decides what to offer.

PDF417

A 2D barcode of stacked rows of bars, wide and short. Found on US driving licences and some boarding passes.

Penalty score

The score an encoder gives each of the eight masks, using four rules about runs, blocks, fake finder patterns and dark-light balance. The mask with the lowest score is used.

Q

QR code

Quick Response code. A 2D barcode designed by Denso Wave in 1994, storing text as a grid of dark and light modules with three finder patterns and built-in error correction.

QRLJacking

An attack on services that log you in by scanning a QR code on a screen. The attacker shows you their login code; you scan it and hand them your session.

Quiet zone

The clear margin around a code, four modules wide in the standard, that lets the scanner see the edge of the finder patterns. Crowding it is the most common reason a code fails to scan.

QRLjacking

Hijacking a login that uses a QR code, such as a messaging app’s “scan to sign in” screen. The attacker shows a victim a live copy of the code; when the victim scans it, the attacker’s browser is logged in. Defence is to scan login codes only on sites you opened yourself.

Quishing

Phishing by QR code. A malicious code, often a sticker over a genuine one on a parking meter or poster, leads to a fake page that asks for card details or a login.

R

Redirect

A server’s answer saying “go to this other address instead”. Dynamic QR codes work by pointing at a redirect service that sends the phone on to the real destination.

Reed-Solomon

The error-correcting code used by QR codes, CDs, DVDs and space probes. It computes repair codewords from the data so that a limited number of wrong codewords can be found and fixed.

Remainder bits

A few leftover modules in some versions that do not fill a whole codeword. They are set to zero and carry nothing.

rMQR

Rectangular Micro QR, standardised as ISO/IEC 23941 in 2022. From 7 by 43 to 17 by 139 modules, holding up to 361 digits. For narrow labels; thin phone support.

S

Scannability

How reliably a code decodes in real conditions. Depends on module size, contrast, quiet zone, print quality and how much of the error-correction budget a logo has already spent.

Separator

The one-module light border around each finder pattern that keeps it distinct from the data beside it.

Short link

A brief web address, such as https://getqrstudio.app/scan?c=dyn, that a redirect service resolves to a longer one. Dynamic codes store a short link, which is why they are smaller.

SQRC

A Denso Wave code with a public part any reader can decode and a private part readable only by specific scanners. Denso Wave does not present it as a security guarantee.

Static QR code

A code holding the destination itself. It needs no server or account and never expires, but cannot be edited or counted after printing.

Structured append

A header allowing one message to be split across up to 16 codes that a reader joins together. Industrial scanners support it; phone camera apps do not.

SVG

Scalable Vector Graphics, the file format to export a code for print. Vector shapes stay sharp at any size, unlike a PNG, which is a fixed grid of pixels.

T

Terminator

The 4-bit sequence 0000 that marks the end of the data segments, before padding begins.

Timing pattern

A row and a column of alternating dark and light modules running between the finder patterns. They let the scanner count modules and so learn the version.

Total scans

In analytics, the count of every request to a dynamic code’s redirect, including repeat scans by the same phone. Compare unique scans.

U

Unique scan

In analytics, a scan from a phone not seen before for that code, judged by a hash of its network address and user agent. A trend line, not an exact headcount.

Universal Link

Apple’s verified deep link. A normal https address that iOS opens in the app that has proved it owns the domain, or in Safari if the app is not installed.

UPC

Universal Product Code, the 12-digit striped barcode on groceries in North America; EAN-13 is its 13-digit counterpart elsewhere. A product number only; the price is in the till’s database.

User agent

A short text a browser sends with each request naming itself and its operating system. Analytics use it to tell iOS from Android from desktop. It does not identify a person.

UTM parameter

A tag added to the end of a web address, such as ?utm_source=qr, so website analytics can tell where a visit came from. The way to measure a static code.

V

vCard

The standard contact-card format, beginning BEGIN:VCARD, that phones import into their address book. Larger than a MeCard but more consistently supported.

Verifiable credential

A digitally signed statement about a person or thing, such as a diploma or a boarding pass, that a verifier can check without calling the issuer. Often presented as a QR code from a digital wallet.

Version

The size grade of a QR code, from 1 (21 modules a side) to 40 (177). Each step adds 4 modules a side. The encoder normally picks the smallest version that fits the payload.

Version information

Eighteen bits, stored twice, that state the version in codes of version 7 and above. Smaller codes have none; the scanner works the version out from the module count.

W

WCAG

The Web Content Accessibility Guidelines. Their contrast rules are a sensible target for a QR code’s colours too: a contrast ratio of at least 4.5:1 between dark and light modules.

Wi-Fi payload

A string such as WIFI:T:WPA;S:NetworkName;P:password;; that a phone recognises and offers to join the network. Text, not a link, so it works with no signal.