Derivative Houses in astrology is a concept when you mentally "move" or "make" another house the new starting point of a question (i.e., the First House). In other words, if you are talking to a client and they ask you if you see anything in their chart about their spouse's health. That is a Derivative question, so you would look to the 7th house as the new 1st house of the question about the client's spouse's health. 
Robert explains using that example in this episode. 

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Thomas Miller's Daily "Fun Astrology" podcast is available here: https://open.spotify.com/show/5tGKH9LJQx7rIWI3pjYjSqAnd his "Subconscious Mind Mastery" podcast is available here: https://open.spotify.com/show/7vbEvOoFU50THEgVzMQnso
Thank you for listening! Robert & Thomas