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Companions were the Doctor's closest friends. Such people knew the Doctor's "secret": that he was someone non-human who travelled in space and time in a police box-shaped craft called the TARDIS. They often directly saved his life, or provided him with a perspective that prevented him from abusing his Time Lord powers. On some occasions, they were the proximate reason that the Doctor sacrificed his then-current life and regenerated. On others, they lost their lives in pursuit of the Doctor's goals.
Because the term was a shortening of the expression "travelling companion", most companions did in fact voyage with him in the TARDIS. Depending on the situation, other words were used to describe the same relationship — most frequently, "assistant". Both terms were occasionally troublesome. Sometimes, the word companion was interpreted sexually, leading to confusion about the Doctor's relationship with his friends. Sometimes, companions bickered over the appropriateness of the word assistant. Rose Tyler said emphatically, "I'm not his assistant", when Sarah Jane called her one. Rose seemed happier being called a "companion", as when Harriet Jones called out over the Sub-Wave Network for "former companions of the Doctor".
