C programs of a particular type are known
as ''big wish'' programs.  They have this
name because of their similarity to the
[wish] program that is part of Tk.

Unless you are completely new to Tcl/Tk,
you are familiar with the program [tclsh].
With [tclsh] you can interactively enter
Tcl commands and see the results of their
evaluation.  Or you can name a script file
for [tclsh] to evaluate.  The Tcl interpreter
within [tclsh] starts out with all the
built-in commands provided by the Tcl package.

The program [wish] is quite similar to [tclsh].
The primary difference is that the Tcl interpreter
within [wish] starts out with all the 
commands provided by both the Tcl and Tk packages.  The
commands of two packages are combined in one interpreter
right from the startup of the program.

A ''big wish'' program is one like [wish] that combines
Tcl and one or more packages in one program.  Examples include:

   * ''expect'' - includes Tcl and [Expect] packages
   * ''expectTk'' - includes Tcl, Tk, and [Expect] packages
   * ''tcl'' - includes Tcl and [TclX] packages
   * ''itclsh'' - includes Tcl and [Itcl] packages

...and there are others.

In the early days of Tcl, the construction of a ''big wish''
program was the only way to extend Tcl with commands
provided by an [extension].

One challenge in those days was if you had a Tcl script
that made use of commands provided by a package other than
Tcl, you had to be careful to choose the right ''big wish''
to evaluate that script, so that it had access to all the
commands it used.  For example, if your script used the
[signal] command, it would need to be run with the ''tcl''
program, and not with the ''tclsh'' program, because [signal]
comes from [TclX].

Tcl 7.5 (released 1996) introduced the [load] command.
This allowed C-coded [extension]s to be added to a Tcl
interpreter dynamically at run time.

As of 2003, these ''big wish'' programs are falling out
of favor.  Replacing them is the practice of making each
component package available as a [load]able extension, so
that each package can be pulled into the [tclsh] program
at runtime, as they are demanded by scripts evaluating
[[package require]] commands.  This is an example of
the extending solution winning out in the [embedding vs. extending]
contest.