Why the same quantity is measured differently by region, and what to watch when converting.
How long a metre is was not set by nature but agreed by people. Different eras and regions made different agreements, and those survive side by side today. Conversion exists not because the arithmetic is hard but because people working from different agreements read the same document.
Errors usually come from assumptions rather than formulas. Units sharing a name while differing in value, mixing weight with volume, and treating a scale whose zero is not absolute as a ratio are the common cases.
The same value can be written in decimal, binary or hexadecimal. None is correct in itself; each suits a different purpose. Colours are written in hexadecimal, permissions in octal and flags in binary because those notations fit those jobs best.
Inside a computer, characters and colours are stored as numbers. The agreement about which number means which character is a character encoding, and the agreement about which number means which colour is a colour space. Broken text and colours that look wrong almost always come from two sides using different agreements.
When a conversion looks wrong, start with the order of magnitude. Being off by around a thousand suggests a prefix, by around double suggests a definition such as radius versus diameter, and by a fixed addition suggests a zero point. Knowing a rough value in advance tells you immediately when a result is wrong.
Inches and centimetres appear in the same document. Where each is used.
Temperature cannot be converted by multiplying, because the zero points differ.
They are just other ways of writing the same number. So why bother?
Sometimes characters show as question marks or strange symbols. What went out of alignment.
Three ways to write the same colour. Which notation suits which situation.
1000 and 1024 are both in use. The misunderstandings that follow from notation.
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