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How to clean horizontal blinds
Keep the blinds slightly open.
Turn the stem until the blinds are mostly closed. The slats should be pointed at a downward angle toward you.
It's important that the blinds remain partially open to allow you clean deep in between them. You won't be able to fit the entire brush attachment into the blinds themselves, but the bristles will penetrate into their inner recesses and sweep the dust into the hose.