The letter 
and 
are the two letters which are articulated from the deepest part of tongue.
The letter which is one of the bold letters as well is pronounced and articulation from deepest most part of our tongue when pronouncing it will touch its opposite part which is the soft palate the back roof of the mouth.
The letter which is one of the soft letters and is pronounced and articulated from deepest part of tongue as well but when pronouncing this the deepest part of the tongue touches hard palate not the soft pallet and is closer to mouth than .
Want to perfect Tajweed rules
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Tajweed Mistakes in these two letters
There is more than 1 mistake that non-Arab make when pronouncing there two letters. First I would like to discuss the letter it is one of the soft letter and the common mistake while pronouncing it is mixing the articulation points with or reading it boldly (hardly) when we pronounce it the air should flow not blocked while we touch the back most part of the thong to the hard palate Now let us see the second word when pronouncing it the most common mistakes is two when articulating it while reading Quran. The first is reading it softly and exchanging its articulating point with toward the hard palate or near it like an English (k) and the second most common mistake is pronouncing it from the top part of thought where 
is pronounced.