In WebDewey, when a number is displayed in the "hierarchy box," above it are displayed its parent, its parent's parent, its parent's parent's parent, etc., while below it are displayed all the numbers for which it is the parent. Sometimes you might be a little surprised by what you see or don't see there. This is particularly the case when spans are involved -- and more particularly the case when overlapping spans are involved.
For example, when 220-290 is displayed, we find above it 200; we find below it 220, 230-280, and 290.
When 230-280 becomes the focal notation, we find above it 200 and 220-290; we find below it 230-270 and 250-280.
When 230-270 becomes the focal notation, we find above it 200, 220-290, and 230-280; we find below it 230 and 240.
The situation at 230-270 (as compared to the situation at 250-280) may seem odd, but it is, in fact, correct. After all, there is an entry for 230-270 Specific elements of Christianity, and its parent is 230-280, as shown.
But why are 250, 260, and 270 not shown as children of 230-270? Because a DDC notation can have only one parent designated in the MARC classification record (153 field, subfield e). The two spans, 230-270 and 250-280, have equal claim as candidate parents for 250 and 260; the two spans, 230-270 and 270-280, have equal claim as candidate parents of 270. But in each case only one of the candidates can be designated the parent. It so happens that 250-280 has been designated the parent of both 250 and 260, while 270-280 has been designated the parent of 270.
The problem arises because of overlapping spans. The graphic below shows the overlapping of spans that occurs in the 200s:
As can be seen, 230-270 overlaps with both 250-280 and 270-280. Fortunately, there are very few cases of overlapping spans. But when you see something in a hierarchy that at first glance looks odd, consider whether any spans are involved, and remember that any DDC number or span of numbers can have only one parent.