来自短语on less (than), 在少于的情况下，后由于该短语的否定含义，介词on被改写成了un.词义演变比较despite, in spite of .

1. The targets would not be achieved unless other departments showed equal commitment.

除非其他部门同样努力，否则这些目标无法达成。

2. Training is worthless unless there is proof that it works.

除非能证明训练有效，否则就是白搭。

3. The South African mailboat arrived on Friday mornings unless bad weather intervened.

除非受恶劣天气干扰，南非邮船一般在星期五上午抵达。

4. All bitches should be spayed unless being used for breeding.

除非是用作种犬，所有的母犬都应摘除卵巢。

5. I tend to forget things unless I mark them down.

除非把事情记下来，否则我常常会忘事。