Focal points—Within the pattern areas of loops and whorls are enclosed the focal points which are used to classify them. These points are called delta and core.
The delta is that point on a ridge at or in front of and nearest the center of the divergence of the type lines.
It may be:
● A bifurcation
● An abrupt ending ridge
● A dot
● A short ridge
● A meeting of two ridges
● A point on the first recurving ridge located nearest to the center and in front of the divergence of the type lines.
The concept of the delta may perhaps be clarified by further exposition. Webster furnishes the following definition: "(1) Delta is the name of the fourth letter of the Greek alphabet (equivalent to the English D) from the Phoenician name for the corresponding letter. The Greeks called the alluvial deposit at the mouth of the Nile, from its shape, the Delta of the Nile. (2) A tract of land shaped like the letter "delta," especially when the land is alluvial, and enclosed within two or more mouths of a river, as the Delta of the Ganges, of the Nile, of the Mississippi" (fig. 19).