Window = {}

-- The prototype with default values
Window.prototype = {x = 0, width = 100, height = 100}

-- Window Meta Table
Window.mt = {}

-- Sets field/table 'o' with the metatable Window. The constructor function
function Window.init(o) setmetatable(o, Window.mt)
  return o
end

--  Metatable Window.mt now has an __index field. This field takes a table and key and returns the appropriate key from the prototype
Window.mt.__index = function (table, key) return Window.prototype[key] end
-- Window.mt.__index = Window.prototype

-- table 'w' now gets the metatable Window.mt
w = Window.init{x = 10, y = 20}

-- table 'w' cannot find a width field so it then checks its metatable Window.mt, Window.mt has the field __index that will take the requested missing key and return a value from the prototype
print(w.width)