Greek Letters
A handful of Greek letters slipped into the original DOS code page because they doubled as math and engineering symbols: pi, sigma and a few others used in formulas long before Unicode existed. The rest of the alphabet, all twenty-four letters in both cases, only became typeable once Unicode added its dedicated Greek block.
These see constant use outside actual Greek text: physics and engineering notation, statistics, and fraternity and sorority names all draw on this page.
Questions
What is the Alt code for pi?
Alt+227 for lowercase pi, one of the original DOS math codes.
Is there an Alt code for capital sigma?
It works inconsistently across keyboard layouts. The HTML entity Σ or the Unicode value U+03A3 is more reliable.
Do lowercase and capital Greek letters need different codes?
Yes. Each case is a separate Unicode character with its own code point and, where one exists, its own Alt code.
Why do scientists and engineers use Greek letters instead of full words?
A Greek letter gives a short, unambiguous symbol for a specific quantity, resistance, wavelength, a rate of change, that would otherwise need a longer, easily confused name.
Is there a difference between the Greek letter omega and the ohm sign?
They look alike but are separate characters. The ohm sign, U+2126, exists for older compatibility. Modern text normally uses the Greek capital omega, U+03A9 (Alt+234), for both purposes.