Once the breath has been released and the sound started, it may be possibleto play a succession of notes without the tongue interrupting the supply ofbreath. The result is an extremely smooth legato, and a group of notes playedthus is marked by a slur. But a legato effect can also be obtained by the use of aless explosive consonant than 't' to start the sound, such as 'd' or 'dh'. 'Legatotonguing' implies individually tongued notes but played as smoothly aspossible. Here it has to be added that tonguing techniques vary according tothe nature of the instrument. Legato tonguing, for example, is common withbrass instruments because a completely slurred legato would often beimpracticable, if not impossible.When notes are to be tongued separately but very rapidly (e.g. insemiquaver passages at fast speeds), more elaborate methods oftonguing areneeded. A succession of notes produced by the tip of the tongue - 'tttttt' etc. -would soon become tiring, so a 'k' sound produced with the back of the tongueis used in alternation: 'tktktk' etc. (double tonguing) or, in groups of three,'tkttkt' etc. (triple tonguing). Double and triple tonguing also make it possiblefor very rapid repetitions of a single note to be performed.An unusual kind of tonguing, occasionally required by 20th-centurycomposers, is created by rolling an 'r' on the tip of the tongue. This technique,known as 'flutter-tonguing', is generally indicated by the use of tremolo signs(e.g. J )and/or in words: the German Flatterzunge (abbreviated as Flzg.) isoften used. Flutter-tonguing is particularly suited to flue instruments such asthe flute, but it can be successful on others too, such as the trumpet andtrombone.