The Romans, the ones who crucified Christ, are known to have used instruments of various shapes. Some were simply upright timbers, others had crosspieces attached either at the top or just a little below the top.
The word "cross" in the New Testament comes from the Greek word stauros. The study of word origins shows that stauros simply means "stake" or "pole." This word was used in literature in reference to pieces of wood of various shapes, including those with crosspieces.
Interestingly, other biblical writers describe the instrument of Christ's death as a "tree." Notice Acts 5:30: "The God of our fathers raised up Jesus whom you murdered by hanging on a tree." Also, Acts 13:29-30: "Now when they had fulfilled all that was written concerning Him, they took Him down from the tree and laid Him in a tomb." Finally, Peter writes: ". . . who Himself bore our sins in His own body on the tree, that we, having died to sins, might live for righteousness. . ." (I Peter 2:24; see also Acts 10:39; Galatians 3:13). Whether the use of this term is more descriptive than literal cannot be determined, but under time restraints as occurred during Jesus' trial and crucifixion, the Romans are known to have used living trees as stauron for crucifixions.
There is no description in the Bible of the specific stauros on which Christ died. If it were important for us to know its shape, God would have provided us with additional information. The important thing is Christ's sacrifice for us and what He is doing now, not the exact shape of the wood on which He died. The mere fact that the traditional cross figures so prominently in pagan religious custom ought to give us pause for thought.