QR code and barcode glossary

Short, checked definitions of the terms that come up when you make, print or specify a QR code or a barcode, from module and quiet zone to GTIN and bearer bar. Every entry links to the tool or guide on this site where the detail lives.

0–9

1D vs 2D barcode #

A linear (1D) barcode encodes data in the widths of bars and spaces along one axis, holds a short number or code, and can be read by a laser scanner. A 2D code — a matrix like QR, Data Matrix and Aztec, or a stacked symbol like PDF417 — encodes in both directions, holds far more, adds error correction and needs an imager or a camera. If the scanners on a floor are laser scanners, no 2D symbology is available until the hardware changes.

A

Alignment pattern #

The smaller concentric squares inside a QR code, present from version 2 upward and multiplying as the symbol grows. They give the decoder reference points across the grid so it can correct perspective distortion when the code is photographed at an angle or printed on a curved surface. Like the finder and timing patterns they are fixed structure with no error-correction cover, so a logo must not sit on one.

Aztec Code #

A 2D matrix symbology whose finder pattern is a bullseye at the centre, so the decoder orients it from the inside out and it needs no quiet zone at all. Sizes run from 15 × 15 to 151 × 151 modules plus a Compact form, holding up to 3,832 digits, 3,067 letters or 1,914 bytes, with Reed–Solomon correction configurable from roughly 5 % to 95 %. It is the code behind rail and air tickets and reads well from phone screens. Standard: ISO/IEC 24778.

B

Bearer bar #

The heavy black frame, or the rules above and below, around an ITF-14 symbol. It stops a scanner from accepting a partial read when the beam clips the top or bottom edge, which matters for Interleaved 2 of 5 because a truncated scan can look like a valid shorter number. Mandatory on ITF-14, with the quiet zone of ten narrow-bar widths kept outside it; optional on plain Interleaved 2 of 5, and worth adding when the symbol is scanned by hand.

C

Check digit #

A final character computed from the rest of the number so a scanner can detect a misread. EAN-13, EAN-8, UPC-A and ITF-14 use modulo 10 with alternating weights of 1 and 3; Code 128 carries a modulo 103 check character that is computed automatically and not printed; Code 39 has an optional modulo 43 digit and is self-checking without it; Interleaved 2 of 5 may add a modulo 10 digit. The generator computes retail check digits for you and rejects a wrong one rather than correcting it.

Code 128 #

The workhorse linear barcode of shipping and warehousing. It encodes all 128 ASCII characters through three subsets, A, B and C, with subset C packing digit pairs so numeric data stays short, at any length the label width allows. A modulo 103 check character is computed automatically and not shown in the human-readable line; the quiet zone is ten narrow-bar widths on each side. With an FNC1 and Application Identifiers it becomes GS1-128. Standard: ISO/IEC 15417.

Code 39 #

An older linear barcode encoding digits, capital letters, space and - . $ / + %, framed by an asterisk as start and stop character that some renderers print in the human-readable line. Lowercase letters are not available. It is self-checking, so the modulo 43 check digit is optional and should only be added when a specification asks for it. Wide for its content; keep the narrow bar above about 0.19 mm for handheld laser scanners. Standard: ISO/IEC 16388.

D

Data Matrix #

A square 2D matrix symbology, in current use always as ECC 200 with Reed–Solomon correction recovering roughly 25 % damage. Sizes run from 10 × 10 to 144 × 144 modules plus six rectangular formats, holding up to 2,335 alphanumeric characters, 3,116 digits or 1,556 bytes, and it needs only a one-module quiet zone. Its L-shaped finder along two edges and small footprint make it the code for direct part marking and labels a few millimetres across. Standard: ISO/IEC 16022.

dpi #

Dots per inch, the resolution of a printer or of a raster image placed for print. Pixels needed for a PNG are centimetres divided by 2.54 times the target dpi: about 591 for 5 cm at 300 dpi, the usual print target, and the API’s 1024-pixel PNG is about 8.7 cm at 300 dpi — beyond that, supply an SVG. On label printers dpi sets the X-dimension: 203 dpi gives 0.125 mm dots, 300 dpi 0.085 mm.

E

EAN-13 #

The retail barcode on most products sold outside North America. It encodes a 13-digit GTIN-13 — GS1 company prefix, article reference and a modulo 10 check digit — in bars whose nominal size is 37.29 × 25.93 mm at 100 % magnification, scalable from 80 % to 200 %. The longer guard bars and the > mark at the right edge are functional: the > marks the quiet zone. Give the generator 12 digits and it appends the check digit. Standard: ISO/IEC 15420.

Error correction level (L, M, Q, H) #

The amount of redundant data a QR code carries so a damaged symbol still decodes: roughly 7 % at L, 15 % at M, 25 % at Q and 30 % at H. Higher levels need more modules, so at the same printed size the pattern gets finer and harder to read. M is the sensible default for a plain code; H is the level for a code with a logo over its centre. The percentage applies to codewords, not to the finder patterns, which nothing protects.

F

Finder pattern #

The fixed shape a decoder looks for first to locate and orient a symbol. In a QR code it is the three large nested squares in three corners; Data Matrix uses a solid L-shaped border along two edges; Aztec puts a bullseye in the centre, which is why it needs no quiet zone. Finder patterns are structure, not data, so error correction does not cover them: a sticker or a logo over one corner makes a QR code unreadable at any level.

G

GS1, Application Identifier, FNC1 #

GS1 is the organisation that issues company prefixes and writes the rules for retail and logistics barcodes. An Application Identifier is a numeric prefix — for a GTIN, a batch number, an expiry date, a serial — that labels each field inside a payload, and FNC1 is the special character at the start that tells a scanner the data follows GS1 rules. Code 128 with both becomes GS1-128; Data Matrix with both becomes GS1 Data Matrix.

GTIN #

Global Trade Item Number, the GS1 article number that a retail barcode encodes. GTIN-13 is what an EAN-13 carries, GTIN-12 a UPC-A, GTIN-8 an EAN-8 and GTIN-14 an ITF-14 on a shipping case. The number comes from a GS1 company prefix and cannot be invented: an invented one is rejected by the retailer or collides with a real product somewhere in the world. A UPC-A is an EAN-13 with a leading zero, so the two are the same GTIN written at different lengths.

H

Human-readable interpretation #

The digits or text printed under a barcode so a person can type the number when the scan fails. Its layout is part of the specification for retail codes: under an EAN-13 the first digit stands outside the left guard bars and the > at the right marks the quiet zone; a UPC-A prints its number-system and check digits outside the guard bars. Code 128’s check character is never shown; Code 39’s start and stop asterisks sometimes are.

I

Interleaved 2 of 5 #

A dense numeric-only linear barcode, usually shortened to ITF, that encodes digits in pairs: the first digit of each pair in the bars, the second in the spaces, which is why the data length must be even. Nothing marks the middle of the symbol, so a scan that starts or ends part-way can return a plausible truncated number; fixed lengths, an optional modulo 10 check digit and bearer bars are the defences. Quiet zone ten narrow-bar widths each side. Standard: ISO/IEC 16390.

ITF-14 #

The GS1 barcode for shipping cases and outer cartons. It encodes a 14-digit GTIN-14 — an indicator digit, the item’s GTIN without its check digit, and a new modulo 10 check digit — in Interleaved 2 of 5 wrapped in a mandatory bearer bar, so it survives being printed directly onto corrugated board. A nominal symbol is about 142 mm wide and 32 mm tall including the bearer bar; GS1 allows 50 % to 100 % on a label. Standard: ISO/IEC 16390 with the GS1 General Specifications.

M

MECARD #

A compact contact format that originated on Japanese handsets: a single line of semicolon-separated keys such as N, TEL and EMAIL. It needs roughly a third fewer characters than the same contact as a vCard, which gives a smaller code, but it has no room for several vCard fields and not every device imports it. Use it only when trimming fields cannot make a vCard code printable; vCard 3.0 is otherwise the safer choice.

Micro QR #

A smaller variant of the QR code, defined for tiny items where even a version 1 symbol takes too much room. It is a separate symbology with its own rules, not a QR code printed small. This generator and the image API produce standard QR codes only, so every QR code described on this site means the full symbol with three finder patterns and a four-module quiet zone.

Module #

The smallest unit of a 2D code: one of the little squares in a QR code or Data Matrix, either dark or light. Everything else is measured in modules — the quiet zone, the version, the minimum print size. In offset or digital print a module should be at least about 0.4 mm across, on thermal labels and rough stock closer to 0.6 mm; multiply that by the module count per side to get the smallest honest print size.

N

Numeric, alphanumeric and byte mode #

The three everyday ways a QR code packs characters. Numeric mode stores digits most compactly, alphanumeric mode covers digits, capital letters and a few symbols, and byte mode takes anything, including lowercase letters and UTF-8 text. The mode decides how much fits: version 40 at level L holds 7,089 digits, 4,296 alphanumeric characters or 2,953 bytes. Byte mode counts octets, so 1,500 letters ä are 3,000 bytes; the generator’s ceiling at level M is about 2,300 bytes.

P

PDF417 #

A stacked linear 2D symbology: 3 to 90 rows of 1 to 30 data columns, each codeword 17 modules of four bars and four spaces. One symbol holds up to 1,850 text characters, 2,710 digits or 1,108 bytes with Reed–Solomon correction at levels 0 to 8, which is why it carries whole records — driving licences, boarding passes, shipping documents. Quiet zone two modules on every side; print at 300 dpi or better. Standard: ISO/IEC 15438.

Q

Quiet zone #

The blank margin a scanner needs around a symbol to find its edges. It is part of the code, not decoration, and cropping it is one of the most common reasons a valid code fails. QR needs four modules on every side, Data Matrix one, PDF417 two and Aztec none; Code 128, Code 39 and Interleaved 2 of 5 need ten times the narrow-bar width at each end. The image API defaults to a margin of 2 and warns below the four the specification asks for.

R

Reed–Solomon #

The error-correcting code that lets a decoder rebuild missing or misread data from redundant codewords appended to the payload. QR codes, Data Matrix ECC 200, PDF417 and Aztec all use it: QR at four selectable levels, Data Matrix with a fixed scheme that recovers roughly 25 % damage, PDF417 at levels 0 to 8 and Aztec anywhere from about 5 % to 95 %. Redundancy protects data, not structure: a covered finder pattern defeats every level.

S

SSCC #

Serial Shipping Container Code, the GS1 number that identifies one logistics unit — a pallet or a case — on its transport label, and what a carrier means by an SSCC label specification. It travels in a GS1-128 barcode: a Code 128 with a leading FNC1 and the Application Identifier for the SSCC in front of the number. In German logistics the same number is called the NVE.

Static vs dynamic QR code #

A static code encodes the destination literally, so the URL is physically in the printed pattern and nothing needs to keep running for it to work. A dynamic code encodes a vendor’s short link that redirects each scan wherever you currently point it, in exchange for a subscription; stop paying and the printed codes die. The symbols are the same technology. Everything made here is static; for an editable destination, encode a path on your own domain and redirect it yourself.

SVG vs PNG #

PNG is a grid of pixels: scaled up in a layout, module edges go soft and the code scans worse. SVG is vector, one unit per module, and stays sharp at any size, so it is the format for anything printed larger than a business card; PNG is right for screens and email. The generator’s logo overlay is composited on a canvas and therefore exists only in the PNG. The image API caps PNG at 1024 px and SVG at 2048.

Symbology #

The rulebook that turns data into a pattern of bars or modules and back: which characters are allowed, how long the data may be, how the check digit is computed, how wide the quiet zone must be. QR Code, EAN-13 and Data Matrix are symbologies; a PNG of one of them is not. The barcode generator here covers twelve, from Code 128 and the EAN/UPC family to Data Matrix, PDF417 and Aztec, each with its own page and rules.

T

Timing pattern #

A single row and a single column of alternating dark and light modules running between the finder patterns of a QR code. The decoder reads it to establish the module grid — how many modules the symbol has per side and where each one begins. It is structural, not data, so it lies outside the error correction; a design element or frame that interrupts it means the grid is misread and the code is dead.

U

UPC-A #

The North American retail barcode, encoding a 12-digit GTIN-12. Its nominal size is 37.29 × 25.91 mm at 100 %, scalable from 80 % to 200 % exactly as for EAN-13, and the two are one system: a UPC-A converted to EAN-13 simply gains a leading zero. The number-system digit and the check digit are printed outside the guard bars by specification. With 11 digits the generator appends the check digit; with 12 it validates yours.

UTM parameter #

Query-string tags — utm_source, utm_medium, utm_campaign, utm_content and utm_term — that analytics tools read to attribute a visit. They matter for printed codes because a camera app opening a link sends no referrer, so untagged scans appear as direct traffic. The convention is utm_medium=qr with utm_source naming the placement. Tags cost modules: a 28-character URL is a 29-module symbol, the same link with four tags is 117 characters and 45 modules.

V

vCard #

The standard text format for a contact record, and what a business-card QR code usually contains. This generator writes vCard 3.0, the version every phone, mail client and CRM imports without help; the newer 4.0 scans less reliably. Name, phone, email, organisation and a URL are the fields worth encoding; a photo is not, because a QR code holds at most 2,953 bytes in total. The record is stored in the pattern, so a printed card cannot be updated later.

Verifier (ISO/IEC 15415 grade) #

A verifier is a calibrated instrument that scores a printed symbol against a standard rather than reporting whether one phone happened to read it. For 2D codes ISO/IEC 15415 measures contrast, modulation, fixed pattern damage, axial and grid non-uniformity and unused error correction, and reports the worst of them as a grade from 4.0 (A) down to 0 (F). Print buyers and retailers specify a minimum grade; a phone test on final stock is the affordable substitute.

Version (QR code) #

The size class of a QR symbol, from version 1 at 21 × 21 modules to version 40 at 177 × 177, each step adding four modules per side. The encoder picks the smallest version that holds the data plus its error correction, so a longer URL or a higher correction level pushes the code up a version and, at a fixed printed size, shrinks every module. Version 40 holds up to 7,089 digits, 4,296 alphanumeric characters or 2,953 bytes at level L.

W

WIFI: format #

The plain-text string behind a Wi-Fi QR code: WIFI:T:WPA;S:network;P:password;; with fields for the security type (T), the SSID (S), the password (P) and an optional H:true for hidden networks. It began in the ZXing barcode library and became a de-facto standard when Android and iOS adopted it. Backslash, semicolon, comma, colon and quote inside the SSID or password must be escaped with a backslash. The password sits in the code in clear text.

X

X-dimension (narrow bar) #

The width of the narrowest bar in a linear barcode, or of one module in a 2D code. It is the unit everything else scales from: quiet zones are quoted as multiples of it, and the wide-to-narrow ratio depends on it. On a printer it has to be a whole number of dots. A 203 dpi thermal printer lays down 0.125 mm dots, so realistic narrow bars are 0.25 mm or 0.375 mm; a 300 dpi printer gives 0.085 mm dots and finer control.

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