Sigma is the only Greek letter in ordinary modern use with two distinct lowercase forms depending on its position in a word: the familiar sigma is used everywhere except at the end of a word, where Greek instead uses a separate final form, a distinction fixed by the Byzantine minuscule reform of roughly the 9th century CE and preserved in Greek spelling ever since, unlike most other Greek letters, which kept one lowercase shape regardless of position. That positional split has no equivalent among the Latin letters covered elsewhere in this atlas, though Hebrew final forms, also covered in this atlas, follow a broadly similar principle.
Modern mathematics uses sigma in both its cases for entirely separate technical purposes: a capital sigma denotes summation, adding together a sequence of terms, a notation introduced in the 18th and 19th centuries, while a lowercase sigma denotes standard deviation in statistics, a measure of how spread out a set of values is around its mean.
The Six Sigma quality-management methodology, developed at Motorola in the 1980s, takes its name directly from that statistical sense of the letter. In the Milesian numeral system, discussed elsewhere in this atlas, sigma carries the value two hundred, a role wholly separate from either of its modern mathematical senses.