The availability of source code

Executable code

Machine, binary language.

00101101 01101100 00101101 01101100 01010011 11001011 01010011 11001011
00011001 00111100 00011001 00111100 11000101 10100101 11000101 10100101
00101101 01101100 00101101 01101100 01010011 11001011 01010011 11001011
00011001 00111100 00011001 00111100 11000101 10100101 11000101 10100101

Source code

Human readable language.

#include <stdio.h>
int main(void) {
  int count;
  for (count=1 ; count<=500 ; count++) {
    printf("I shall not throw paper planes in class.\n");
  }
  return 0;
}

Source code -> binary code = Compilation

Binary code -> Source code = Reverse engineering, often forbidden => black box

The availability of source code is a necessary condition (but not a sufficient condition) for Free software.