Reference articles on the subjects behind the tools.
These are reference articles rather than tool documentation. Each one covers a subject the site actually had to understand properly in order to build something — how a barcode encodes a number, what a JPEG throws away, what a PDF is underneath, why a shuffle is usually wrong — and each is written to be useful whether or not you ever open the tool it sits beside.
They are deliberately not sales pages. Where something has a real limitation, the article says so, and where a common piece of advice is wrong, it explains why rather than repeating it.
A plain-English walkthrough of EAN-13 and UPC-A: modules, guard patterns, the three digit alphabets, the hidden 13th digit, and how the check digit works.
How JPEG, PNG and WebP actually shrink a picture: chroma subsampling, the DCT, quantisation, generation loss, and why resizing first beats turning the quality slider down.
Entropy explained with real numbers: why length beats symbol substitution, how passphrases compare, what offline cracking really costs, and why reuse matters more than complexity.
Inside the PDF format: objects, streams and the cross-reference table, why merging is lossless and cheap, why compressing is not, and why some PDFs are searchable and others are not.
Client-side versus server-side processing explained, how to verify a site's privacy claims yourself with browser developer tools, and what still leaves your device regardless.
UTC versus offsets, why daylight saving creates hours that happen twice and hours that never happen, how the IANA database works, and the rules for scheduling across zones.
Inside the QR symbol: finder patterns, timing and alignment, Reed-Solomon error correction, masking, and why you can put a logo in the middle without breaking it.
Pseudo-random versus cryptographic generators, why modulo introduces bias, why sorting by a random comparator is not a shuffle, and how to make a draw genuinely fair.
What JSON does and does not allow, why large integers silently lose precision, how duplicate keys behave, and why there is no date type.
Why temperature cannot be scaled like length, the two different gallons and pints, fluid ounces versus ounces, three kinds of ton, and the kilobyte that is not 1000 bytes.
RGB and hex, why sRGB values are not linear light, why HSL lightness does not match perceived brightness, and how WCAG contrast ratios are actually calculated.
How words per minute is actually defined, why reaction-time tests measure your hardware as well as you, and what a browser can and cannot honestly measure.
Mostly people who have hit a specific problem and want to understand it rather than work around it: a barcode that will not scan, a photograph that looks blocky after saving, a PDF that refuses to be searched, a reminder that fired an hour late in March. Each guide starts from the mechanism, because once you can see the mechanism the practical rules stop being arbitrary and start being obvious.
No prior knowledge is assumed beyond ordinary familiarity with using a computer. Where arithmetic helps, it is worked through with real numbers rather than left as an exercise. Where a subject has genuine depth — and most of these do — the guide goes into it rather than stopping at the summary, on the assumption that someone reading about check digits actually wants to know how check digits work.
Building a tool forces you to learn a subject to a level that using one never does. A barcode generator has to know that EAN-8 weights its check digit differently from EAN-13, because getting that wrong produces labels that scan as the wrong product. An image compressor has to know what chroma subsampling does, because that is why red text turns to mush. A time zone converter has to know that some local times occur twice a year and others never occur at all.
That knowledge is usually left buried in the code. These articles are an attempt to write it down properly, in the belief that the explanation is worth as much as the tool — and occasionally more, since understanding why a file is large is more durable than any particular button that makes it smaller.
Specific claims here are ones that can be checked: the module counts in a barcode symbol, the weighting used by a check digit, the entropy of a five-word passphrase, the reason a redaction rectangle fails to redact. Worked examples use real numbers and can be verified by hand.
Where something is a judgement call rather than a fact — which JPEG quality setting is acceptable, how many words a passphrase should be — the guide says so and explains the trade-off, instead of presenting a preference as a rule.
If something here is wrong, out of date, or unclear, please say so through the contact page. Technical subjects drift — time zone rules change several times a year, image formats gain browser support, guidance on passwords has shifted substantially over the past decade — and a correction is more useful than a compliment.