Who set up the second round interview for you? HR or the hiring manager? Either way, ask the one who set up the second round of interview for you - whom you will meet with and etc.
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Also, it is depending on what position you are applying for. For example, when we hire someone for the manager position, here is the list of various rounds of interviews the candidates go through:
1. HR - phone screen
2. First round of on site interview - to meet with future team members, direct report manager, and peer managers;
3. Second round of on site interview - to meet with future direct manager's manager, and his peer managers
4. Third round of on site interview - optional, and normally only the final 1 or 2 candidates will get here. To meed the division general manager. Just to make sure we do not hire some one the big boss hates :-)