There are 7 letters which are called Huroof Musta'liyah. They are:

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خ  ص  ض  غ  ط  ق  ظ

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An easier way to remember these letters is to say them together: خُصَّ ضَغْطٍ قِظْ

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All Huroof Musta'liyah letter should always be read full mouth.