A variable that is defined in a let or function definition can be seen and used by all of the code inside that definition. This is called lexical scoping.

It is possible to have defines inside a function to define sub-functions that can only be used within that function. This can make the code clearer and reduce the number of parameters that must be passed:

(define (mysqrt x)
  (let ((epsilon 1.0e-8))

    (define (mysqrtb estimate)

      (define (new-estimate)
        (/ (+ estimate
              (/ x estimate))
           2))

      (if (good-enough x (square estimate)
                       epsilon)
          estimate
          (mysqrtb (new-estimate)) ) )

  (mysqrtb 1.0) ) )  ; the code of mysqrt

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