σίγμα

The Phoenician letter shin (𐤔) meant "tooth" and looked like a jagged W. Greeks adopted it around 800 BCE, rotated and simplified it, and called it sigma (Σ, σ). It represented the /s/ sound. The capital form Σ had sharp angles; the lowercase σ was rounder. Greek also developed a final form, ς, used only at the end of words.

In the 18th century, Leonhard Euler began using Σ as the notation for summation—adding up a series of terms. The choice was natural: sigma is the Greek equivalent of S, and S stands for "sum." Euler's notation became universal. Every mathematics student learns to read Σᵢ₌₁ⁿ as "the sum from i equals 1 to n."

In the 1890s, Karl Pearson adopted the lowercase σ for standard deviation—the measure of how spread out data is from the mean. The connection to summation is direct: standard deviation is calculated using a sum of squared differences. Sigma became statistics' shorthand for variation, uncertainty, and noise.

In 1986, engineer Bill Smith at Motorola created Six Sigma—a quality methodology demanding that manufacturing defects fall within six standard deviations of the mean, allowing only 3.4 defects per million. Jack Welch made it the centerpiece of General Electric's strategy in the 1990s. A Phoenician tooth, a Greek letter, a mathematical symbol, and finally a corporate philosophy: sigma's journey is as varied as the meanings it accumulated.