I have seen in a church a cross with a loop off the top that makes the vertical look like a P. Does this symbol have a name? What is its meaning? From what does it derive? Can you supply any more interesting information about it? The loop was to my right, and was more flag-like than a P, so drooping a bit.
Actually, this symbol is a "tau-rho," not a "chi-rho." Instead of being a shortened form of the word "Christ" (Χριστος), generally speaking the Tao-Rho is thought to be a shortened form of the words "cross" and "crucify" (σταυρος, σταυροω). Larry Hurtado has speculated that it may also be the first pictoral representation of Jesus on a cross (the loop of the "P" is the head). See Hurtado's The Earliest Christian Artifacts pp. 135ff, he calls it a "staurogram."
Answering my own question, I found this, which is different: Staurogram Christliche Symbolik (Menzel) I 193 2.jpg
The Staurogram (meaning monogram of the cross, from the Greek: ΣTAΥPOΣ meaning cross), or Monogrammatic Cross or Tau-Rho symbol, is composed by a tau (Τ) superimposed on a rho (Ρ). The Staurogram was first used to abbreviate the Greek word for cross in very early New Testament manuscripts such as P66, P45 and P75, almost like a nomina sacra.
Ephrem the Syrian in the 4th-century explained these two united letters stating that the tau refers to the cross, and the rho refers to the Greek word "help" (Greek: Βoηθια) which has the numeric value of 100 as the letter rho has. In such a way the symbol expresses the idea that the Cross saves. The two letters tau and rho can also be found separately as symbols on early Christian ossuaries: the tau was considered a symbol of salvation due to the identification of the tau with the sign which in Ezechiel 9:4 was marked on the forehead of the saved ones, or due to the tau-shaped outstretched hands of Moses in Exodus 17:11. The rho by itself can refer to Christ as Messiah because Abraham, taken as symbol of the Messiah, generated Isaac according to a promise made by God when he was one hundred years old, and 100 is the value of rho.
So my take-away is that the symbol derives from save/salvation and cross/Christ, or Christ saves (or perhaps faith in Christ saves).