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+# Expressions
+
+Every AST node in SQLGlot is represented by a subclass of `Expression`. Each such expression encapsulates any necessary context, such as its child expressions, their names, or arg keys, and whether each child expression is optional or not.
+
+Furthermore, the following attributes are common across all expressions:
+
+#### key
+
+A unique key for each class in the `Expression` hierarchy. This is useful for hashing and representing expressions as strings.
+
+#### args
+
+A dictionary used for mapping child arg keys, to the corresponding expressions. A value in this mapping is usually either a single or a list of `Expression` instances, but SQLGlot doesn't impose any constraints on the actual type of the value.
+
+#### arg_types
+
+A dictionary used for mapping arg keys to booleans that determine whether the corresponding expressions are optional or not. Consider the following example:
+
+```python
+class Limit(Expression):
+    arg_types = {"this": False, "expression": True}
+
+```
+
+Here, `Limit` declares that it expects to have one optional and one required child expression, which can be referenced through `this` and `expression`, respectively. The arg keys are generally arbitrary, but there are helper methods for keys like `this`, `expression` and `expressions` that abstract away dictionary lookups and related checks. For this reason, these keys are common throughout SQLGlot's codebase.
+
+#### parent
+
+A reference to the parent expression (may be `None`).
+
+#### arg_key
+
+The arg key an expression is associated with, i.e. the name its parent expression uses to refer to it.
+
+#### comments
+
+A list of comments that are associated with a given expression. This is used in order to preserve comments when transpiling SQL code.
+
+#### type
+
+The data type of an expression, as inferred by SQLGlot's optimizer.