Difference between sight letter of credit and usance?

In simplest Words, A sight LC means that whenever the Bill of Exchange is presented at counter, it must be paid then and there (ofcourse if the LC docx are in order).

While a Usance Bill is first presented then accepted and after that it is paid. Usance bills are usually for a period of 30, 60, 90 days.

So if a usance bill is of 30 days then it would be paid 30 days after presentation and acceptance.