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+# Basic Concepts
+
+The most important concepts in the **mp-units** library are `Dimension`, `QuantitySpec`, `Unit`,
+`Reference`, `Representation`, `Quantity`, and `QuantityPoint`:
+
+```mermaid
+flowchart TD
+    Dimension --- QuantitySpec
+    QuantitySpec --- Reference
+    Unit --- Reference
+    Reference --- Quantity
+    Representation --- Quantity
+    Quantity --- QuantityPoint
+    PointOrigin --- QuantityPoint
+
+    click Dimension "#dimension"
+    click QuantitySpec "#quantityspec"
+    click Unit "#unit"
+    click Reference "#reference"
+    click Representation "#representation"
+    click Quantity "#quantity"
+    click PointOrigin "#pointorigin"
+    click QuantityPoint "#quantitypoint"
+```
+
+
+## `Dimension`
+
+`Dimension` concept matches a [dimension](../../../appendix/glossary/#dimension) of either a base
+or derived [quantity](../../../appendix/glossary/#quantity):
+
+- [Base dimensions](../../../appendix/glossary/#base-dimension) are explicitly defined by a user
+  by inheriting from the instantiation of a `base_dimension` class template. It should be instantiated with
+  a unique symbol identifier describing this dimension in a specific
+  [system of quantities](../../../appendix/glossary/#system-of-quantities).
+- [Derived dimensions](../../../appendix/glossary/#derived-dimension) are implicitly created
+  by the library's framework based on the [quantity equation](../../../appendix/glossary/#quantity-equation)
+  provided in the [quantity specification](../../../appendix/glossary/#quantity_spec).
+
+??? abstract "Examples"
+
+    `isq::dim_length`, `isq::dim_mass`, `isq::dim_time`, `isq::dim_electric_current`,
+    `isq::dim_thermodynamic_temperature`, `isq::dim_amount_of_substance`, and
+    `isq::dim_luminous_intensity` are the dimensions of base quantities in the
+    [ISQ](../../../appendix/glossary/#isq).
+
+    IEC 80000 provides `iec80000::dim_traffic_intensity` base dimension to extend ISQ
+    with information technology quantities.
+
+    `derived_dimension<isq::dim_length, per<isq::dim_time>>` is a resulting dimension of
+    `isq::dim_length / isq::dim_time`.
+
+    A `Dimension` can be defined by the user in the following way:
+
+    ```cpp
+    inline constexpr struct dim_length : base_dimension<"L"> {} dim_length;
+    ```
+
+
+## `QuantitySpec`
+
+`QuantitySpec` concept matches all the [quantity specifications](../../../appendix/glossary/#quantity_spec)
+including:
+
+- [Base quantities](../../../appendix/glossary/#base-quantity) defined by a user by inheriting from
+  the `quantity_spec` class template instantiated with a [base dimension](../../../appendix/glossary/#base-dimension)
+  argument.
+- [Derived named quantities](../../../appendix/glossary/#derived-quantity) defined by a user by
+  inheriting from the `quantity_spec` class template instantiated with a result of a
+  [quantity equation](../../../appendix/glossary/#quantity-equation) passed as an argument.
+- Other named quantities forming a [hierarchy of quantities](../../../appendix/glossary/#quantity-hierarchy)
+  of the same [kind](../../../appendix/glossary/#kind) defined by a user by inheriting from the
+  `quantity_spec` class template instantiated with another "parent" quantity specification passed as an
+  argument.
+- [Quantity kinds](../../../appendix/glossary/#kind) describing a family of mutually comparable quantities.
+- Intermediate [derived quantity](../../../appendix/glossary/#derived-quantity) specifications being
+  a result of a [quantity equations](../../../appendix/glossary/#quantity-equation) on other specifications.
+
+??? abstract "Examples"
+
+    `isq::length`, `isq::mass`, `isq::time`, `isq::electric_current`, `isq::thermodynamic_temperature`,
+    `isq::amount_of_substance`, and `isq::luminous_intensity` are the specifications of base quantities
+    in the [ISQ](../../../appendix/glossary/#isq).
+
+    `isq::width`, `isq::height`, `isq::radius`, and `isq::position_vector` are only a few of many
+    quantities of a kind length specified in the [ISQ](../../../appendix/glossary/#isq).
+
+    `kind_of<isq::length>` behaves as any of the quantities of a kind length.
+
+    `isq::area`, `isq::speed`, `isq::moment_of_force` are only a few of many derived quantities provided
+    in the [ISQ](../../../appendix/glossary/#isq).
+
+    `derived_quantity_spec<isq::length, per<isq::time>>` is a resulting quantity specification of
+    `isq::length / isq::time`.
+
+    `QuantitySpec` can be defined by the user in one of the following ways:
+
+    ```cpp
+    inline constexpr struct length : quantity_spec<dim_length> {} length;
+    inline constexpr struct height : quantity_spec<length> {} height;
+    inline constexpr struct speed : quantity_spec<length / time> {} speed;
+    ```
+
+
+## `Unit`
+
+`Unit` concept matches all the [units](../../../appendix/glossary/#unit) in the library including:
+
+- [Base units](../../../appendix/glossary/#base-unit) defined by a user by inheriting from the `named_unit`
+  class template instantiated with a unique symbol identifier describing this unit in a specific
+  [system of units](../../../appendix/glossary/#system-of-units).
+- Named scaled units defined by a user by inheriting from the `named_unit` class template instantiated
+  with a unique symbol identifier and a product of multiplying another unit with some magnitude.
+- Prefixed units defined by a user by inheriting from the `prefixed_unit` class template instantiated
+  with a magnitude and a unit to be prefixed.
+- [Derived named units](../../../appendix/glossary/#derived-unit) defined by a user by inheriting from the
+  `named_unit` class template instantiated with a unique symbol identifier and a result of
+  [unit equation](../../../appendix/glossary/#unit-equation) passed as an argument.
+- [Derived unnamed units](../../../appendix/glossary/#derived-unit) being a result of a
+  [unit equations](../../../appendix/glossary/#unit-equation) on other units.
+
+!!! note
+
+    In the **mp-units** library, physical constants are also implemented as units.
+
+??? abstract "Examples"
+
+    `si::second`, `si::metre`, `si::kilogram`, `si::ampere`, `si::kelvin`, `si::mole`, and `si::candela`
+    are the base units of [SI](../../../appendix/glossary/#si).
+
+    `si::kilo<si::metre>` is a prefixed unit on length.
+
+    `si::radian`, `si::newton`, and `si::watt` are examples of named derived quantities within
+    [SI](../../../appendix/glossary/#si).
+
+    `derived_unit<si::metre, per<si::second>>` is a resulting derived unit of `si::metre / si::second`.
+
+    `non_si::minute` is an example of a scaled unit of time.
+
+    `si::si2019::speed_of_light_in_vacuum` is a physical constant standardized by the SI in 2019.
+
+    `Unit` can be defined by the user in one of the following ways:
+
+    ```cpp
+    inline constexpr struct second : named_unit<"s", kind_of<isq::time>> {} second;
+    inline constexpr struct gram : named_unit<"g", kind_of<isq::mass>> {} gram;
+    inline constexpr struct minute : named_unit<"min", mag<60> * second> {} minute;
+    inline constexpr struct kilogram : decltype(kilo<gram>) {} kilogram;
+    inline constexpr struct newton : named_unit<"N", kilogram * metre / square(second)> {} newton;
+    ```
+
+
+### `AssociatedUnit`
+
+`AssociatedUnit` concept describes a [unit with an associated quantity](../../../appendix/glossary/#associated-unit)
+and is satisfied by:
+
+- All units derived from a `named_unit` class template instantiated with a unique symbol identifier
+  and a [`QuantitySpec`](#quantityspec).
+- All units being a result of a [unit equations](../../../appendix/glossary/#unit-equation) on other
+  associated units.
+
+??? abstract "Examples"
+
+    All units in the [SI](../../../appendix/glossary/#si) have associated quantities.
+
+    Natural units typically do not have an associated quantity.
+
+
+### `PrefixableUnit`
+
+`PrefixableUnit` concept is satisfied by all units derived from a `named_unit` class template for
+which a customization point `unit_can_be_prefixed<U>` was not explicitly set to `false`. Such
+units can be passed as an argument to a `prefixed_unit` class template.
+
+??? abstract "Examples"
+
+    All units in the [SI](../../../appendix/glossary/#si) can be prefixed with SI-defined prefixes.
+
+    Some [off-system units](../../../appendix/glossary/#off-system-unit) like `non_si::day`
+    can't be prefixed.
+
+### `UnitOf<QS>` { #unitof }
+
+`UnitOf<QS>` concept is satisfied for all units matching an [`AssociatedUnit`](#associatedunit)
+concept with an associated quantity type implicitly convertible to `QS`.
+
+
+## `Reference`
+
+`Reference` concept is satisfied by all [quantity reference types](../../../appendix/glossary/#reference)
+types. Such types provide all the meta-information required to create a [`Quantity`](#quantity).
+A `Reference` can be either:
+
+- An [AssociatedUnit](#associatedunit)
+- The instantiation of a `reference` class template with a [`QuantitySpec`](#quantityspec) passed as
+  the first template argument and a [`Unit`](#unit) passed as the second one.
+
+??? abstract "Examples"
+
+    `si::metre` is defined in the [SI](../../../appendix/glossary/#si) as a unit of `isq::length`
+    and thus can be used as a reference to instantiate a quantity of length.
+
+    The expression `isq::height[m]` results with `reference<isq::height, si::metre>` which can be used to
+    instantiate a quantity of `isq::height` with a unit of `si::metre`.
+
+
+### `ReferenceOf<V>` { #referenceof }
+
+`ReferenceOf<V>` concept is satisfied by references that match the following value `V`:
+
+| `V`                  | Condition                                                    |
+|----------------------|--------------------------------------------------------------|
+| `Dimension`          | The dimension of a quantity specification is the same as `V` |
+| `QuantitySpec`       | The quantity specification is implicitly convertible to `V`  |
+| `quantity_character` | The quantity specification has a character of `V`            |
+
+
+## `Representation`
+
+`Representation` concept constraints a type of a number that stores the
+[value of a quantity](../../../appendix/glossary/#quantity-value).
+
+
+### `RepresentationOf<Ch>` { #representationof }
+
+`RepresentationOf<Ch>` concept is satisfied by all `Representation` types that are of a specified
+[quantity character](../../../appendix/glossary/#character). A user can declare a custom representation
+type to be of a specific character by providing the specialization with `true` for one or more of
+the following variable templates:
+
+- `is_scalar<T>`
+- `is_vector<T>`
+- `is_tensor<T>`
+
+
+## `Quantity`
+
+`Quantity` concept matches every [quantity](../../../appendix/glossary/#quantity) in the library and is
+satisfied by all types being or deriving from and instantiation of a `quantity` class template.
+
+??? abstract "Examples"
+
+    All of `42 * m`, `42 * si::metre`, `42 * isq::height[m]` create a quantity and thus satisfy
+    a `Quantity` concept.
+
+    A quantity type can also be specified explicitly (i.e. `quantity<si::metre, int>`,
+    `quantity<isq::height[m]>`).
+
+### `QuantityOf<V>` { #quantityof }
+
+`QuantityOf<V>` concept is satisfied by all the quantities for which a [`ReferenceOf<V>`](#referenceof)
+is `true`.
+
+## `PointOrigin`
+
+`PointOrigin` concept matches all [quantity point origins](../../../appendix/glossary/#point-origin) in
+the library. It is satisfied by either:
+
+- Compile-time known [`QuantityPoint`](#quantitypoint)
+- All types derived from an `absolute_point_origin` class template.
+
+??? abstract "Examples"
+
+    The types of both definitions below satisfy a `PointOrigin` concept:
+
+    ```cpp
+    inline constexpr struct mean_sea_level : absolute_point_origin<isq::height> {} mean_sea_level;
+    inline constexpr auto ice_point = quantity_point<isq::thermodynamic_temperature[K]>{273.15};
+    ```
+
+
+### `PointOriginFor<QS>` { #pointoriginfor }
+
+`PointOriginFor<QS>` concept is satisfied by all [`PointOrigin`](#pointorigin) types that are defined using
+a provided quantity specification.
+
+
+## `QuantityPoint`
+
+`QuantityPoint` concept is satisfied by all types being either a specialization or derived from `quantity_point`
+class template.
+
+??? abstract "Examples"
+
+    The following specifies a quantity point defined in terms of an ice point provided in the previous example:
+
+    ```cpp
+    constexpr auto room_reference_temperature = quantity_point<isq::Celsius_temperature[deg_C], ice_point>{21};
+    ```
+
+
+### `QuantityPointOf<V>` { #quantitypointof }
+
+`QuantityPointOf<V>` concept is satisfied by all the quantity points for which either:
+
+- a [`ReferenceOf<V>`](#referenceof) is `true`.
+- the type of `V` satisfies [`PointOrigin`](#pointorigin) concept and the quantity point's
+  [absolute point origin](../../../appendix/glossary/#absolute-point-origin) is convertible to `V`.
+
+## Interoperability concepts
+
+### `QuantityLike`
+
+`QuantityLike` concept is satisfied by a type `T` for which an instantiation of `quantity_like_traits`
+type trait yields a valid type that provides:
+
+- static member `reference` that matches the [`Reference`](#reference) concept
+- `rep` type that matches [`RepresentationOf`](#representationof) concept with the character provided
+  in `reference`
+- `number(T)` static member function returning a type convertible to `rep`
+
+??? abstract "Examples"
+
+    This is how support for `std::chrono::seconds` can be provided:
+
+    ```cpp
+    template<>
+    struct quantity_like_traits<std::chrono::seconds> {
+      static constexpr auto reference = si::second;
+      using rep = std::chrono::seconds::rep;
+      [[nodiscard]] static constexpr rep number(const std::chrono::seconds& q) { return q.count(); }
+    };
+    ```
+
+### `QuantityPointLike`
+
+`QuantityPointLike` concept is satisfied by a type `T` for which an instantiation of
+`quantity_point_like_traits` type trait yields a valid type that provides:
+
+- static member `reference` that matches the [`Reference`](#reference) concept
+- static member `point_origin` that matches the [`PointOrigin`](#pointorigin) concept
+- `rep` type that matches [`RepresentationOf`](#representationof) concept with the character provided
+  in `reference`
+- `relative(T)` static member function returning a type that can be used to construct
+  the [`QuantityPoint`](#quantitypoint) type
+
+??? abstract "Examples"
+
+    This is how support for a `std::chrono::time_point` of `std::chrono::seconds` can be provided:
+
+    ```cpp
+    template<typename C>
+    struct quantity_point_like_traits<std::chrono::time_point<C, std::chrono::seconds>> {
+      static constexpr auto reference = si::second;
+      static constexpr auto point_origin = absolute_point_origin<isq::time>;
+      using rep = Rep;
+      [[nodiscard]] static constexpr auto relative(const std::chrono::time_point<C, std::chrono::seconds>& qp)
+      {
+        return std::chrono::duration_cast<std::chrono::seconds>(qp.time_since_epoch());
+      }
+    };
+    ```