In the tented arch, most of the ridges enter upon one side of the impression and flow or tend to flow out upon the other side, as in the plain arch type; however, the ridge or ridges at the center do not. There are three types of tented arches: - The type in which ridges at the center form a definite angle; i.e., 90 deg. or less. - 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; i.e., 45 deg. or more. - The type approaching the loop type, possessing two of the basic or essential characteristics of the loop, but lacking the third. Figures 122 to 133 are examples of the tented arch. [Illustration: 122] [Illustration: 123] [Illustration: 124] [Illustration: 125] [Illustration: 126] [Illustration: 127] [Illustration: 128] [Illustration: 129] [Illustration: 130] [Illustration: 131] [Illustration: 132] [Illustration: 133] Figures 122 to 124 are of the type possessing an angle. Figures 125 to 129 reflect the type possessing an upthrust. Figures 130 to 133 show the type approaching the loop but lacking one characteristic. Tented arches and some forms of the loop are often confused. It should be remembered by the reader that the _mere converging of two ridges does not form a recurve, without which there can be no loop_. On the other hand, there are many patterns which at first sight resemble tented arches but which on close inspection are found to be loops, as where one looping ridge will be found in an almost vertical position within the pattern area, entirely free from and passing in front of the delta. Figure 134 is a tented arch. The ridge marked "A--A" in the sketch enters on one side of the impression and flows to the other with an acute rise in the center. Ridge C strikes into A at point B and should not be considered as a bifurcating ridge. The ridges marked "D--D" would form a tented arch if the rest of the pattern were absent. [Illustration: 134] [Illustration: 135] Figure 135 is a sketch of a pattern reflecting a ridge, A--B, entering on one side of the impression, recurving, and making its exit on the other side of the impression. The reader should study this sketch carefully. It should be borne in mind that there must be a ridge entering on one side of the impression and recurving in order to make its exit on the same side from which it entered, or having a tendency to make its exit on that side, before a pattern can be considered for possible classification as a loop. This pattern is a tented arch of the upthrust type. The upthrust is C. There is also an angle at E. D cannot be termed as a delta, as the ridge to the left of D cannot be considered a type line because it does not diverge from the ridge to the right of D but turns and goes in the same direction.