To discuss where a new class might go, it is helpful to have a map of the current classes. The following is the basic class hierarchy of gnu Smalltalk. Indentation means that the line inherits from the earlier line with one less level of indentation.1.
Object Behavior ClassDescription Class Metaclass BlockClosure Boolean False True Browser CFunctionDescriptor CObject CAggregate CArray CPtr CCompound CStruct CUnion CScalar CChar CDouble CFloat CInt CLong CShort CSmalltalk CString CUChar CByte CBoolean CUInt CULong CUShort Collection Bag MappedCollection SequenceableCollection ArrayedCollection Array ByteArray WordArray LargeArrayedCollection LargeArray LargeByteArray LargeWordArray CompiledCode CompiledMethod CompiledBlock Interval CharacterArray String Symbol LinkedList Semaphore OrderedCollection RunArray SortedCollection HashedCollection Dictionary IdentityDictionary MethodDictionary RootNamespace Namespace SystemDictionary Set IdentitySet ContextPart BlockContext MethodContext CType CArrayCType CPtrCType CScalarCType Delay DLD DumperProxy AlternativeObjectProxy NullProxy VersionableObjectProxy PluggableProxy File Directory FileSegment Link Process SymLink Magnitude Association Character Date LargeArraySubpart Number Float Fraction Integer LargeInteger LargeNegativeInteger LargePositiveInteger LargeZeroInteger SmallInteger Time Memory Message DirectedMessage MethodInfo NullProxy PackageLoader Point ProcessorScheduler Rectangle SharedQueue Signal Exception Error Halt ArithmeticError ZeroDivide MessageNotUnderstood UserBreak Notification Warning Stream ObjectDumper PositionableStream ReadStream WriteStream ReadWriteStream ByteStream FileStream Random TextCollector TokenStream TrappableEvent CoreException ExceptionCollection UndefinedObject ValueAdaptor NullValueHolder PluggableAdaptor DelayedAdaptor ValueHolder
While initially a daunting list, you should take the time to hunt down the classes we've examined in this tutorial so far. Notice, for instance, how an Array is a subclass below the SequenceableCollection class. This makes sense; you can walk an Array from one end to the other. By contrast, notice how this is not true for Sets: it doesn't make sense to walk a Set from one end to the other.
A little puzzling is the relationship of a Bag to a Set, since a Bag is actually a Set supporting multiple occurrences of its elements. The answer lies in the purpose of both a Set and a Bag. Both hold an unordered collection of objects; but a Bag needs to be optimized for the case when an object has possibly thousands of occurrences, while a Set is optimized for checking object uniqueness. That's why Set being a subclass or Bag, or the other way round, would be a source of problems in the actual implementation of the class. Currently a Bag holds a Dictionary associating each object to each count; it would be feasible however to have Bag as a subclass of HashedCollection and a sibling of Set.
Look at the treatment of numbers—starting with the class Magnitude. While numbers can indeed be ordered by less than, greater than, and so forth, so can a number of other objects. Each subclass of Magnitude is such an object. So we can compare characters with other characters, dates with other dates, and times with other times, as well as numbers with numbers.
Finally, you will have probably noted some pretty strange classes, representing language entities that you might have never thought of as objects themselves: Namespace, Class and even CompiledMethod. They are the base of Smalltalk's “reflection” mechanism which will be discussed later, in The truth on metaclasses.
 This listing is courtesy of the printHierarchy method supplied by gnu Smalltalk author Steve Byrne. It's in the kernel/Browser.st file.