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src/docs/overview.md

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 title: "Overview"
 ---
 
+## DD4hep 
+
+DD4hep introduces the concept of a `DetElement` which is related to the volumes (logical volumes) and placed volumes but not in a strict way.
+These elements build up a separate tree which describes the relationship between the volumes and the readout detectors. 
+Through the unique BitField ID this allows position space information to be mapped to and from channel number/cell ID space.
+
+![DetElement](DD4hep_block_diagram.svg)
 
 ## EIC Data model