The spin state of a silver atom in the Stern–Gerlach experiment can be represented in a two state space. The spin can be aligned with a measuring apparatus (arbitrarily called 'up') or oppositely ('down'). In Dirac's notation these two states can be written as | u ⟩ ⟩ , | d ⟩ ⟩ . The space of a two spin system has four states, | u u ⟩ ⟩ , | u d ⟩ ⟩ , | d u ⟩ ⟩ , | d d ⟩ ⟩ .