IHS
For the folks quizzing me on the letters on the altar vestments on Saturday:
The IHS is a symbolic monogram of Christ.... This monogram consists of the Greek letters iota, eta, and sigma, the first three letters of the name Iesous (Greek for Jesus), the letters of which are also used to spell out the phrase "Iesous Hominum Salvator," "Jesus, savior of man." It relates to the story of Constantine, whose vision of the Chi-Rho was recorded by Church Father Eusebius. In the vision, Constantine was reported to have heard a voice proclaim, "In this symbol, thouse shalt conquer." Therefore, the IHS has also stood for "In Hoc Signo," in this sign.
http://altreligion.about.com/library/glossary/symbols/bldefsihs.htm
The IHS is a symbolic monogram of Christ.... This monogram consists of the Greek letters iota, eta, and sigma, the first three letters of the name Iesous (Greek for Jesus), the letters of which are also used to spell out the phrase "Iesous Hominum Salvator," "Jesus, savior of man." It relates to the story of Constantine, whose vision of the Chi-Rho was recorded by Church Father Eusebius. In the vision, Constantine was reported to have heard a voice proclaim, "In this symbol, thouse shalt conquer." Therefore, the IHS has also stood for "In Hoc Signo," in this sign.
http://altreligion.about.com/library/glossary/symbols/bldefsihs.htm