how to use the phrase 'to which'? what does that mean?

Question

Answer

We use "to which" so that we can avoid ending a sentence with the preposition "to."  So instead of saying, "Which office should I send this to?" we can say, "To which office should I send this?"

Answer

Maybe some examples can help you clarify this doubt:

1. To which address should I send the invoice? = Where should I send the invoice?
2. I don't know yet to which department I will assign this task, but I will think about it during the weekend.