The Side of the Tongue
There are two letters that use the side of the tongue for articulation.  

They do not use the same part of the side of the tongue, rather the uses the posterior two thirds of the side (or edge) of the tongue, and the uses the anterior one third (closest to the mouth) portion of the side of the tongue.

The letter

This letter is articulated from the one or both sides of the tongue and from the molars and the gum area next to the molars.  The posterior two thirds of the side of the tongue is used for this letter.  This letter can be articulated from one side (right or left) of the tongue alone, or from both sides of the tongue simultaneously.  

has the characteristic of compression of the sound, as well as tafkheem (heaviness), so the deep part of the tongue  raises up when pronouncing it and compresses the sound at the same time.

Common mistakes in the letter

The     takes practice, patience, and dua’ to perfect its articulation.  The most common mistake in its pronunciation is using the tip of the tongue instead of the side (posterior two thirds).  The resultant sound then is that of a .  Using the middle of the tongue and what opposes it of the middle of the roof of the mouth is another common error.  The resultant sound is sort of like a heavy .

Some Arabs even sometimes have difficulty pronouncing correctly.  Some areas of the Arab world consistently pronounce this letter using the tip of the tongue and the teeth and the resultant sound is exactly that of a .  This is most noted in Sudan. 

Another less common mistake is of going too far back to articulate it; back to the throat, in this case the sound is similar to a swallowing sound mixed with a letter.

The letter

This letter has the widest articulation points of all the Arabic letters.  It is articulated from the anterior one third of the sides of the tongue until the sides end at the tip, and what lies opposite to them of the gums of the two top front incisors, the two top lateral incisors, the two top canines, and the two top premolars.  The articulation of this letter is then in the shape of an arc, with both sides until the tongue ends at the tip meeting up with with gums of all the mentioned teeth.  It is to be noted that it is a fine line of the sides of the tongue that meets up with the gums, and it does not include the top of the tongue.  The articulation of the is an upward movement, not a forward movement like “L” is in English. 

Common mistakes in the letter

The most common mistake is that of using just the tip and not the sides of the tongue to pronounce this letter, and the resultant sound is that of a noon, since the noon is articulated very close to the .  Another mistake for native English speakers is pronouncing using the underside of the tongue and the actual plates of the two front teeth.  The resultant sound is a heavy sound, just as the English “L”.