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diff --git a/modelcif/descriptor.py b/modelcif/descriptor.py
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+"""Classes to describe the chemistry of custom chemical components.
+
+   If a given :class:`ihm.ChemComp` is not defined in either the wwPDB
+   chemical component dictionary (CCD) or the ModelArchive CCD, its
+   chemistry can be described with one or more of these objects. They
+   are passed as the ``descriptors`` argument when creating a new
+   :class:`ihm.ChemComp`.
+"""
+
+
+class Descriptor:
+    """Base class for all descriptors.
+       This class is generally not used directly; instead, a subclass
+       such as :class:`IUPACName` or :class:`InChI` is employed.
+
+       :param str value: The actual name or identifier describing
+              the chemistry.
+       :param str details: Additional details about this descriptor.
+       :param software: The software used to generate the descriptor, if any.
+       :type software: :class:`modelcif.Software`
+    """
+
+    def __init__(self, value, details=None, software=None):
+        self.value, self.details = value, details
+        self.software = software
+
+    def __repr__(self):
+        return "<%s(%s)>" % (self.__class__.__name__, repr(self.value))
+
+
+class CanonicalSMILES(Descriptor):
+    """Simplified Molecular-Input Line-Entry System (SMILES) computed from
+       chemical structure devoid of isotopic and stereochemical information."""
+    type = 'Canonical SMILES'
+
+
+class IsomericSMILES(Descriptor):
+    """Simplified Molecular-Input Line-Entry System (SMILES) computed from
+       chemical structure containing isotopic and stereochemical information.
+
+       SMILES written with isotopic and chiral specifications are collectively
+       known as isomeric SMILES."""
+    type = 'Isomeric SMILES'
+
+
+class IUPACName(Descriptor):
+    """Chemical name computed from chemical structure that uses International
+       Union of Pure and Applied Chemistry (IUPAC) nomenclature standards."""
+    type = 'IUPAC Name'
+
+
+class InChI(Descriptor):
+    """International Chemical Identifier (InChI) computed from chemical
+       structure using the International Union of Pure and Applied Chemistry
+       (IUPAC) standard."""
+    type = 'InChI'
+
+
+class InChIKey(Descriptor):
+    """International Chemical Identifier hash (InChIKey) computed from
+       chemical structure using the International Union of Pure and Applied
+       Chemistry (IUPAC) standard."""
+    type = 'InChI Key'
+
+
+class PubChemCID(Descriptor):
+    """PubChem Compound ID."""
+    type = 'PubChem CID'