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6. The law if one does not know how to blow one of the sounds?
The sounds of the Tekiah and [the middle sound of] Teruah [or Shevarim] invalidate each other [if one of them is not sounded]. Thus if one knows how to blow the Tekiah but does not know how to blow [neither] the Teruah [or the Shevarim] then he should not blow at all. If however one knows how to blow the [Tekia and] Teruah but does not know how to blow the Shevarim, then he should blow the set of Teruah three times [Tarat 3x]. Similarly if he knows how to blow the [Tekia and ] Shevarim but does not know how to blow the Teruah, then he should blow the set of Shevarim three times [Tashat 3x]. [However, in such a case one does not recite a blessing prior to the blowing. The same applies to one that does not have enough breath to blow more than one set.]
 593:3; Michaber 593:2
 The reason: As there is no Mitzvah involved in simply blowing a Tekiah or Teruah or Shevarim as the Mitzvah is to blow a Tekiah and then a Teruah or Shevarim and then a Tekiah, as explained in 590:1. [Admur ibid]
 The reason: As perhaps the Shevarim or Teruah is the correct middle blow of the Torah and hence he fully fulfills the Mitzvah by blowing it. [Admur ibid; M”A 593:3]
If one knows Tashrat: See Michaber ibid that writes “if one knows how to blow Tashrat but not Tashat and Tarat”; The Mefarshim discuss the obvious question on this wording as certainly if he knows Tashrat he knows the other blows. Various answers are given to this question: 1) The Michaber means to say he does not know how to blow Tashrat in one breath [Taz 593:6] 2) He is unable to blow more than Tashrat. [M”A 593:3] See Nehar Shalom 593; Mamar Mordechai 593:1; Kaf Hachaim 593:6; To note that Admur completely negates this issue and omits the set of Tashrat form this Halacha.
 M”E 593:2; Biur Halacha 593; Kaf Hachaim 593:5
 M”A 593:3