1.  Xsī is the fourteenth letter in the Greek alphabet. It's English spelling may be "xi" or "xsi." This grammar follows the latter to reflect its compound consonant's sound.

2.  Xsī is a compound consonant. Its phoneme is a combination of κ + [unvoiced] σ. Its phoneme is approximate to "x" in "axe" or in "six."

3.  Xsī's lower and upper case letters are very different. The lower case letter is distinctive with a curl stroke at the top and bottom.

4.  Xsī is transliterated into English as "Xs" and "xs" or "X" and "x."