There are a combination of letters on a table that sits at the front of First Presbyterian in Golden that look like this:
I puzzled over it for a while and finally asked Earl Palmer from University Presbyterian in Seattle.
One of the things I like about talking with Earl is that he just doesn’t give you the answer. For instance, Earl could have just said, “It means Iota Eta Sigma.” But no. He explained that it’s one of the Greek words for Jesus, coming from the Greek word Î™Î·ÏƒÎ¿Ï…Ï‚ (Iesous) which translates to ‘Jesus’, but is often just shortened to Î™Î·Ïƒ. To get from Î™Î·Ïƒ to IHS, make the former all upper case letters: Î™Î—Î£. Almost there, the last thing you need to know is that Î£ is the letter Sigma. So, take the ‘S’ from Sigma and you have IHS.
Earl then went on to explain Î§ÏÎ¹ÏƒÏ„Î¿ (Christos) that means Christ (natch) and is often abbreviated to Î§Ï.
And finally, Î‘Î© (Alpha/Omega) which refers to Revelation 22:13:
I am the Alpha and the Omega, the Beginning and the End, the First and the Last.
Here’s the site I used to lookup the exact Greek definitions/spellings of what Earl gave me: http://www.htmlbible.com/sacrednamebiblecom/kjvstrongs/STRINDEX.htm
Wikipedia also has an article on the Christogram.