Same letter, different case, and a completely different job: one names a quantity, the other tells you to do something.
Case decides it. Lowercase σ is a variable — standard deviation in statistics, stress in engineering. Capital Σ is an operator: it instructs you to add a series up, and will normally carry limits above and below it. If it has an index underneath, it is the summation sign.
The capital of this letter is the summation sign. One names a quantity, the other instructs an operation, and readers routinely take the two for one symbol.
Drawn from the visual attributes recorded for each symbol. Only traits the two records disagree about are listed.
| Trait | Sigma | Summation sign |
|---|---|---|
| Built from | letter like and circles | letter like and abstract |
| Symmetry | none | horizontal |
| Sigma | Summation sign | |
|---|---|---|
| What it does | Names a quantity | Instructs an operation — add these up |
| How to tell them apart | Lowercase, and stands alone | Capital, usually with limits above and below |
| Typical reading | Standard deviation, stress, conductivity | The sum from i=1 to n of… |
| Unicode | U+03C3 | U+2211 |
| Relationship (from Symbolwise's records) | Commonly confused with Summation sign — The capital of this letter is the summation sign. One names a quantity, the other instructs an operation, and readers routinely take the two for one symbol. | Commonly confused with Sigma — This sign is the capital Greek letter sigma. The lowercase letter names a quantity — standard deviation, stress — while this capital instructs an operation, and readers routinely take the two for one symbol. |
Unicode Code Chart: Greek and Coptic (U+0370–U+03FF)
The Unicode Consortium · retrieved 2026-07-30
Authoritative for the character's code point, name and block only. Unicode assigns identity; it does not define what a symbol means in use.
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for the widely-agreed uses of the letter across fields and for its history. Not used to assert that any one use is the letter's definitive meaning — no body defines that, and the record says so.
Wikipedia · retrieved 2026-07-30
Tertiary reference. Used for widely-agreed identity and origin summary, not for contested detail or exact dates.
Alpha is a Greek letter and stands for a quantity — an angle, a coefficient, a significance level. The proportionality sign is an operator and stands between two quantities to say they keep a constant ratio. The drawings differ in that the proportionality sign's right-hand stroke runs out in an open curve rather than closing into the letter's tail.
The becquerel counts nuclear decays — one per second — and says nothing about harm. The sievert weights absorbed energy by the kind of radiation delivering it, and so estimates biological effect. A becquerel of one isotope and a becquerel of another can carry very different risk, which is exactly why exposure limits and medical doses are stated in sieverts and never in becquerels.
Look at what follows. A prefix never stands alone — it is glued to a unit symbol, as in kW or kg. A constant stands by itself in an equation. Careful physical writing subscripts the constant as k_B for exactly this reason, and the kelvin's capital K is a third claimant to the same letter.
Equal-length lines mean a capacitor. Alternating long and short lines mean a battery, with the long line positive. If the second line is curved rather than straight it is a polarised capacitor — still a capacitor, but one that must be fitted the right way round.