int fieldVar = 20;                  // field variable.
final static int FIELD_CONST = 15;  // field constant.

void setup() {
  int localVar = 30 + fieldVar;  //accessing a field variable
  println(localVar);
  println(fieldVar);

  // localVar ceases to exist when its scope ends.
  // fieldVar exists for the whole sketch class.
}

void draw() {
  background(-1);
  rect(fieldVar, height>>1, FIELD_CONST, FIELD_CONST);
  if (++fieldVar == width)  fieldVar = -FIELD_CONST;

  // both width & height are also field variables,
  // which sketch inherited from class PApplet.
}