ζ | zeta

The Greek letter ζ (zeta) was derived from the ancient Phoenician letter 𐤆 (zayin) which meant “sword” or some other weapon.

In the system of Greek numerals, the letters α to ε have the respective values 1 through 5. Bizarrely, ζ has the value of 7. This may account for the fact that chemists don’t like using Greek letters beyond ε when numbering the atoms in the chain. Still, the lower-case letter ζ has some uses in maths and sciences, for example:

• in colloidal chemistry: ε_i, ζ-potential, the electrokinetic potential of colloidal dispersions
• in mathematics: ζ(s), the Euler–Riemann zeta function
• in mathematics: ζ(z; Λ), the Weierstrass zeta function
• in pharmacology: ζ-opioid receptor, aka opioid growth factor receptor

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七 | qī

In Mandarin Chinese, 七 (Pinyin: qī 🔊) means “seven”. It is considered a lucky number, maybe because it is pronounced similarly to 气 (qì), “life essence”.

Many historical forms of this character looked like 十, which currently represents “ten”. If you got accustomed to write your 7 with a line through the middle, then you’ll have no problem remembering 七: it’s just 7 upside down.

The Chinese hand sign for “seven” is a hand with the fingertips of the thumb and first two fingers touching:

There is another hanzi with the same meaning, 柒, which is mainly used for financial purposes. It is composed of the very same 七 as well as of now-familiar 氵, a radical form of 水 (shuǐ), and 木 (mù).

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