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