What is the relationship between Brutus and Cassius?

Brutus and Cassius are extremely close friends who have known each other for a very long time. It is exactly because they love each other so intensely that they feel able to speak plainly to one another, even when they disagree. Their fiercest arguments are swiftly resolved. There is a telling moment in Act 4, Scene 3, when Brutus confesses that when he spoke harshly, he was "ill-tempered" and did not mean it; Cassius asks for his hand, and Brutus promises his "heart too." Early in the play, Cassius is convinced that his friend Brutus is the most valuable person he can possibly recruit to his cause. He knows that Brutus does not see himself in such a way but offers to be a "mirror" to show Brutus the qualities in himself which he does not suspect are there.
The relationship between the friends is at times turbulent, with each accusing the other of being less skilled militarily and each seeking to persuade the other to his point of view. Ultimately, however, their friendship is built not only on mutual respect but upon the deepest affection.