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Common Lisp the Language, 2nd Edition
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The set of names of all slots accessible in an instance of a class
C is the union of the sets of names of slots defined by
C and its superclasses. The structure of an instance
is the set of names of local slots in that instance.
In the simplest case, only one class among C and its
superclasses defines a slot with a given slot name. If a slot is defined
by a superclass of C, the slot is said to be
inherited. The characteristics of the slot are determined by
the slot specifier of the defining class. Consider the defining class
for a slot S. If the value of the :allocation slot
option is :instance, then S is a local slot and
each instance of C has its own slot named S that
stores its own value. If the value of the :allocation slot
option is :class, then S is a shared slot, the
class that defined S stores the value, and all instances of
C can access that single slot. If the :allocation
slot option is omitted, :instance is used.
In general, more than one class among C and its superclasses can define a slot with a given name. In such cases, only one slot with the given name is accessible in an instance of C, and the characteristics of that slot are a combination of the several slot specifiers, computed as follows:
:allocation slot option, :instance is used.
Less specific slot specifiers do not affect the allocation.:initform slot option in the most specific slot specifier
that contains one. If no slot specifier contains an
:initform slot option, the slot has no default initial
value form.(and
...
) where 
are the values of the
:type slot options contained in all of the slot specifiers.
If no slot specifier contains the :type slot option, the
contents of the slot will always be of type t. The result
of attempting to store in a slot a value that does not satisfy the type
of the slot is undefined.:initarg slot options in all the slot specifiers.:documentation slot option in the most specific slot
specifier that contains one. If no slot specifier contains a
:documentation slot option, the slot has no documentation
string.A consequence of the allocation rule is that a shared slot can be
shadowed. For example, if a class 
defines a slot named S
whose value for the :allocation slot option is
:class, that slot is accessible in instances of and all of its subclasses.
However, if 
is a subclass of
and also defines a slot
named S, ’s slot is
not shared by instances of
and its subclasses. When a class defines a shared slot, any
subclass of will share this single slot
unless the defclass form for specifies a slot of the same name
or there is a superclass of
that precedes in the class
precedence list of that
defines a slot of the same name.
A consequence of the type rule is that the value of a slot satisfies the type constraint of each slot specifier that contributes to that slot. Because the result of attempting to store in a slot a value that does not satisfy the type constraint for the slot is undefined, the value in a slot might fail to satisfy its type constraint.
The :reader, :writer, and
:accessor slot options create methods rather than define
the characteristics of a slot. Reader and writer methods are inherited
in the sense described in section 28.1.3.1.
Methods that access slots use only the name of the slot and the type
of the slot’s value. Suppose a superclass provides a method that expects
to access a shared slot of a given name, and a subclass defines a local
slot with the same name. If the method provided by the superclass is
used on an instance of the subclass, the method accesses the local
slot.
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