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The residences in the mid-1920s need confirmation.
New York Public Library [2011-02-07]
Universal Edition catalog, 2011
Rocky Mountain Online Archive [2015-04-28]; communication from University of New Mexico John Donald Robb Musical Trust, 2014-04-28, 2015-04-28 (abbreviated below as UNM Robb Musical Trust)
Northwest Digital Archives [2014-06-30]
Dixon, J. 1992
Röntgen did not number his quartets.
The Julius Röntgen website [2012-06-24] says that he settled in the Netherlands in 1878.