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docs/framework/basic_concepts.rst

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+.. namespace:: units
+
+Basic Concepts
+==============
+
+The most important concepts in the library are `Unit`, `Dimension`, and
+`Quantity`:
+
+.. image:: /_static/img/design.png
+    :align: center
+
+`Unit` is a basic building block of the library. Every dimension works with
+a concrete hierarchy of units. Such hierarchy defines a reference unit and
+often a few scaled versions of it.
+
+`Dimension` concept matches a dimension of either a base or derived quantity.
+`base_dimension` is instantiated with a unique symbol identifier and a base
+unit. `derived_dimension` is a list of exponents of either base or other
+derived dimensions.
+
+`Quantity` is a concrete amount of a unit for a specified dimension with a
+specific representation.