The Greek alphabet has three forms for the voiceless alveolar sibilant letter -- SIGMA, two lower case, and one capitalized. The second lower case SIGMA is only used at the end of the word.
These letters are not in our alphabetical order. If you want to know how the Greek alphabet acquired this particular order, please read the "Alphabet Order" section of The Development of the Greek Alphabet.
Even if you never plan to learn Greek, there are good reasons to familiarize yourself with the alphabet. Mathematics and Science use Greek letters like PI (π) to complement the numeric symbols. That same SIGMA in its capital form, can stand for "sum," while the letter DELTA can mean "change."
Some books in English are numbered using the letters of the Greek alphabet. Sometimes, both lower case and capitals are employed for simplification. Thus, you may find that the books of the Iliad are written Α to Ω and those of the Odyssey, α to ω.
Why You Should Know the Greek Alphabet
|Upper Case||Lower Case||Letter Name|