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"The Henle Library app combines G. Henle Publisher’s reliable Urtext and the latest technology – on your tablet or iPad (only). Practice, rehearse and make music – whenever and wherever you like. Make use of the many unique features of the digital Henle Library and go far beyond the printed page. The Henle Library is available in German, English, Chinese, Japanese and Korean. A must for professionals, amateurs, students and teachers."
"The ultimate goal of the IMSLP is to gather all public domain music scores, in addition to the music scores of all contemporary composers (or their estates) who wish to release them to the public free of charge."
Nearly all of the major labels are represented, and new releases tend to get added promptly. Also includes a certain amount of musical theater. Create a personal login; then download one of their mobile apps to take music with you (you can download recordings for offline listening).
On portable devices, Naxos works best by using the Naxos app.