Conversation so far:
What did Ludwig Mies van der Rohe do first in Ludwig Mies van der Rohe's career?
He worked in his father's stone carving shop and at several local design firms.
What design firms did he work for?
He began his architectural career as an apprentice at the studio of Peter Behrens from 1908 to 1912.

Question: What did he create there?
Self-contained rewrite: What did  Ludwig create there?