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Mirror Hall vs Duet Display: catalog facts
Mirror Hall	Duet Display
Free Free and open source under the GPL-3.0 or later.	Paid Paid app with plans listed on the pricing page.
Linux	Windows, macOS, Android, iOS
Turns a spare Linux device into a second screen No extra hardware needed	Uses tablets and phones as extra monitors Drawing tablet mode for illustration
Both devices must run Linux Young project with little documentation	Paid, with features split across plans Requires an account
Downloadable app	Downloadable app
Open source	Closed source
License: GPL-3.0-or-later	License not stated
No account needed	Account needed
Not stated if it works offline	Not stated if it works offline
Official website Full listing and sources Checked October 1, 2026	Official website Full listing and sources Checked October 1, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.