diff --git a/source/specifications/declaring-build-dependencies.rst b/source/specifications/declaring-build-dependencies.rst
index 9e5b72308..3ac94b0b3 100644
--- a/source/specifications/declaring-build-dependencies.rst
+++ b/source/specifications/declaring-build-dependencies.rst
@@ -5,6 +5,99 @@
 Declaring build system dependencies
 ===================================
 
-``pyproject.toml`` is a build system independent file format defined in :pep:`518`
-that projects may provide in order to declare any Python level dependencies that
-must be installed in order to run the project's build system successfully.
+The ``pyproject.toml`` file is written in `TOML <https://toml.io>`_.
+Among other metadata (such as :ref:`project metadata <declaring-project-metadata>`),
+it declares any Python level dependencies that must be installed in order to
+run the project's build system successfully.
+
+.. TODO: move this sentence elsewhere
+
+Tables not defined by PyPA specifications are reserved for future use.
+
+
+build-system table
+------------------
+
+.. TODO: merge with PEP 517
+
+The ``[build-system]`` table is used to store build-related data.
+Initially,  only one key of the table is valid and is mandatory
+for the table: ``requires``. This key must have a value of a list
+of strings representing dependencies required to execute the
+build system. The strings in this list follow the :ref:`version specifier
+specification <version-specifiers>`.
+
+An example ``build-system`` table for a project built with
+``setuptools`` is:
+
+.. code-block:: toml
+
+   [build-system]
+   # Minimum requirements for the build system to execute.
+   requires = ["setuptools"]
+
+Build tools are expected to use the example configuration file above as
+their default semantics when a ``pyproject.toml`` file is not present.
+
+Tools should not require the existence of the ``[build-system]`` table.
+A ``pyproject.toml`` file may be used to store configuration details
+other than build-related data and thus lack a ``[build-system]`` table
+legitimately. If the file exists but is lacking the ``[build-system]``
+table then the default values as specified above should be used.
+If the table is specified but is missing required fields then the tool
+should consider it an error.
+
+
+.. TODO: move elsewhere
+
+tool table
+----------
+
+The ``[tool]`` table is where any tool related to your Python
+project, not just build tools, can have users specify configuration
+data as long as they use a sub-table within ``[tool]``, e.g. the
+`flit <https://pypi.python.org/pypi/flit>`_ tool would store its
+configuration in ``[tool.flit]``.
+
+A mechanism is needed to allocate names within the ``tool.*``
+namespace, to make sure that different projects do not attempt to use
+the same sub-table and collide. Our rule is that a project can use
+the subtable ``tool.$NAME`` if, and only if, they own the entry for
+``$NAME`` in the Cheeseshop/PyPI.
+
+JSON Schema
+-----------
+
+To provide a type-specific representation of the resulting data from
+the TOML file for illustrative purposes only, the following
+`JSON Schema <https://json-schema.org>`_ would match the data format:
+
+.. code-block:: json
+
+   {
+       "$schema": "http://json-schema.org/schema#",
+
+       "type": "object",
+       "additionalProperties": false,
+
+       "properties": {
+           "build-system": {
+               "type": "object",
+               "additionalProperties": false,
+
+               "properties": {
+                   "requires": {
+                       "type": "array",
+                       "items": {
+                           "type": "string"
+                       }
+                   }
+               },
+               "required": ["requires"]
+           },
+
+           "tool": {
+               "type": "object"
+           }
+       }
+   }