An incredibly handy feature of IRB is the ability to use _ to access the result of the last evaluated expression. Perfect for when you just executed a long calculation and realized you forgot to assign it to a variable:

2 + 2
# => 4
_ * 3
# => 12

earth_radius = 6_371
Math.sqrt(2 * Math::PI * earth_radius**2)
# => 20015.086796020572

circumference = _
# => 20015.086796020572
    

History

_ has been around since Ruby 1.6.0, released in August 2000.