换言之 / 简言之【huànyánzhī / jiǎnyánzhī】

换言之 and 简言之 open a clause that restates what came before. 换言之 – "change the words" – says the same thing in other terms, often spelling out what it implies. 简言之 – "brief words" – compresses a longer stretch into a short version. Both are the written, classical-flavoured forms of 换句话说 and 简单地说.

换言之: the same point, made concrete

What follows 换言之 adds no new fact. It makes the previous statement concrete or draws its consequence.

A new fact after 换言之 breaks the link.

• × 他很努力，换言之，他也很聪明 → 他很努力，而且也很聪明

简言之: the short version

简言之 usually follows a long explanation, at the start of a new paragraph or as the last sentence of one.

总之 and 总而言之 also close a passage, gathering several points into one. 简言之 shortens a single point.

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Both belong to writing and formal speech. In conversation, 换句话说, 也就是说 and 简单地说 do the same job.