diff --git a/docs/CMakeLists.txt b/docs/CMakeLists.txt
index efad16e2..28526b94 100644
--- a/docs/CMakeLists.txt
+++ b/docs/CMakeLists.txt
@@ -3,7 +3,7 @@
 xo_doxygen_collect_deps()
 xo_docdir_doxygen_config()
 xo_docdir_sphinx_config(
-    index.rst examples.rst glossary.rst install.rst abstractions.rst
+    index.rst examples.rst glossary.rst install.rst implementation.rst
     quantity-reference.rst quantity-class.rst quantity-factoryfunctions.rst quantity-unitvars.rst
     unit-reference.rst unit-concept.rst unit-quantities.rst
 )
diff --git a/docs/implementation.rst b/docs/implementation.rst
index f23c6a25..a48fe480 100644
--- a/docs/implementation.rst
+++ b/docs/implementation.rst
@@ -1,7 +1,9 @@
-.. _abstractions:
+.. _implementation:
 
-Representation
-==============
+Abstraction Tower
+=================
+
+Abstraction tower for *xo-unit* components.
 
 .. ditaa::
 
@@ -19,7 +21,41 @@ Representation
     |       dimension       |
     +-----------------------+
 
-- quantity: see :doc:`quantity-reference`
+- quantity:    see :doc:`quantity-reference`.
+  quantity with compile-time unit work
+
+- xquantity:
+  quantity with unit work deferred until runtime
+
+- scaled_unit:
+  a unit involving zero or more dimensions, and associated conversion factor.
+
+  - expresses result of arithmetic involving multiple scales.
+  - scaled_units are closed under multiplication and division.
+  - multiplication and division commit to a single :code:`basis_unit` for each
+    dimension.
+
+- natural_unit:
+  a unit involving zero or more dimensions, and at most one scale per dimension.
+  A quantity instance is always represented as a dimensionless multiple
+  of a natural unit
+
+  - natural_units are *not* closed under multiplication and division.
+    (for example consider :code:`xo::qty::qty::foot * xo::qty::qty::meter`)
+
+- bpu:
+  a power of a basis unit. Has a single dimension.
+
+- basis_unit:
+  a unit with a single dimension and scale.
+
+- dimension:
+  identifies a dimension, such as mass or time.
+
+Representation
+==============
+
+Worked example using :cpp:class:`xo::qty::quantity`
 
 .. code-block:: cpp
     :linenos:
diff --git a/docs/index.rst b/docs/index.rst
index a68e6163..f6fa3413 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -14,7 +14,7 @@ however there are some important differences:
 
 * streamlined implementation using c++20 features.
 * along with no-runtime-overhead compile-time unit inference,
-  also provides defer-until-runtime representation. 
+  also provides defer-until-runtime representation.
 * constexpr string representation for things like unit abbreviations.
 * supports fractional dimensions, for concepts like volatility.
 * integration with python (see sister project xo-pyunit)
@@ -32,7 +32,7 @@ runtime (since we can't construct new c++ types at runtime).
 
    install
    examples
-   abstractions
+   implementation
    unit-quantities
    quantity-reference
    unit-reference