This is the last letter in the Greek alphabet.

Psi is written as ψ. It is the sound "ps" as in "lips." It is transliterated as "ps."

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There are four consonants in the Greek alphabet that produce a double sound. They are θ (th), φ (ph), χ (ch), and ψ (ps).

Interesting: Some people like to pronounce χ as xsi. In this case, χ would be a fifth double sound.