The shape of the cross is compatible with the shape of the human back. A large Celtic cross on the back can make a dramatic tattoo, with the shaft of the cross running down the backbone and the horizontal part running across the shoulder blades. Celtic crosses can be made very elaborate with the inclusion of interlocking Celtic knots, and sometimes with background imagery to highlight the main image.
The Chi Rho is a Celtic Christian symbol that looks like an elongated P with an X across the shaft of the P. It is formed by the first two letters in the Greek word for "Christ." One of the most famous renditions of the Chi Rho image is in the "Book of Kells," a very important Celtic manuscript from the eighth century. Like many Celtic images, the Chi Rho is a bold and simple graphic design, so it lends itself well to tattooing. A tattooed Chi Rho symbol can be anything from a tiny reminder on a wrist that can be covered by a watch, to an elaborate back tattoo with multiple colors and integrated personal references.
The four evangelists are represented within Christian iconography by an angel (Matthew), a lion (Mark), a calf (Luke), and an eagle (John). These four images are combined in many creative ways in Christian tattooing. All four symbols lend themselves to dramatic representation, due both to their inherent beauty and because of the ancient social and religious associations that are attached to them. An example of their use in tattooing might be each symbol on an ankle and a wrist, indicating a devotion in walking and work to the lessons taught by the four evangelists. Another use would be the integration of the angel, lion, calf, and eagle into a Celtic cross tattoo, putting each of them into a corner of the cross.