Plain Arch and Tented Arch Patterns
Definition: The ridges enter on one side of the impression and flow or tend to flow out the other with a rise or wave in the center. There may be various ridge formations such as ending ridges, bifurcations, dots, and so forth, but they all tend to follow the general ridge contour. The “crest" of an arch is the highest point reached by the rising friction ridge. Most often that will be in the middle of the friction ridge. The plain arch is also known as an absence of pattern (the plain arch has no delta, no real core as in a loop, nor can a ridge tracing be conducted as in a whorl pattern) (Figures 3.38 through 3.46).
The Plain Arch
Definition: Most of the ridges enter on one side of the impression and flow or tend to flow out on the other side, as in the plain arch; however, the ridge or ridges at the center do not (Figures 3.47 through 3.55).
1. There are three types of tented arches.
a. The type in which ridges at the center form a definite angle of 90 degrees or less (Figure 3.74).
b. The type in which one or more ridges at the center form an upthrust. An upthrust is an ending ridge of any length rising at a sufficient degree from the horizontal plane, that is, 45 degrees or more (Figure 3.75).
c. The type approaching the loop, possessing two of the three basic essential characteristics, but lacking the third.
i. The mere converging of two ridges does not form a recurve, without which there can be no loop.
ii. The presence of the slightest upthrust at the center of the impression is sufficient to make the pattern a tented arch.
iii. The upthrust must be an ending ridge. It cannot be a continuation of a curving ridge. Test for upthrust: If the ridges on both sides of the ending ridge follow direction or flow trend of the upthrust, the print may be classed as a P/А. If, however, the ridges on only one side follow its direction, it is a T/A.
iv. An appendage or spike abutting upon a recurve at right angles in the space between the shoulders of a loop on the outside is considered to spoil the recurve.
Figure 3.47-3.55 Plain arch patterns.