A Dictionary of Music and Musicians/Sostenuto
SOSTENUTO, 'sustained'; a direction which has of late come to be used with a considerable degree of ambiguity. It originally signified that the notes were to be held for their full value, and was thus equivalent to tenuto; but in music of the modern 'romantic' school it very often has the same meaning as meno mosso, or something between that and ritenuto—i.e. the passage so marked is to be played at a uniform rate of decreased speed until the words a tempo occur. No precise rule can be given for its interpretation, as its use varies with different masters, and even in different works by the same master. One of the most remarkable instances of its use is in the Introduction to Beethoven's Symphony No. 7, which is marked Poco sostenuto only, with no direction as to speed. The 'Meeresstille' in his op. 112 is Sostenuto, the Preludium before the Benedictus in the Mass in D is Sostenuto ma non troppo, and the Kyrie of the same work Assai sostenuto. So is the Introduction to the A minor Quartet, op. 132. Here we have all the varieties.
[ J. A. F. M. ]