The words unless and until are often used together but their meanings are different, and often confused.
edit Meaning of Unless vs Until
The word until is used in the context of time and roughly means before and up to the time. For example,
- I cannot get to the TPS reports until next week.
- I lived with my parents until I was 17.
The word unless is used in the context of a precondition and means if this condition is not met. For example,
- I will not shoot that squirrel unless someone puts a gun to my head.
- Peter will not go to the picnic unless his dog is allowed to accompany him.
edit Using the phrase 'unless and until'
The phrase unless and until is commonly used, which often leads to people thinking that the two words mean the same thing. However, this phrase should be used when the situation calls for both a context of time and a precondition. For example, Paul's mother told him that he will not get to play with any of his toys unless and until his homework is completed. In this sentence, completing the homework is a precondition, as well as connotes the time horizon after which Paul is allowed to play with his toys.