In the book Tajweed Rules of the Qur'an Part 2 by Kareema Czerepinski, it is mentioned that the laam merges with the 14 'Shamsee' letters due to the same makhraj, However, what about the letter duad?

Question
As Salaamu alaykum wa rahmatullahi wa barakatuh

In the book Tajweed Rules of the Qur'an Part 2 by Kareema Czerepinski, it is mentioned on Page 40 that the laam merges with the 14 'Shamsee' letters due to the same makhraj. However, what about the letter duad? This is also a shamsiya letter, but the makhraj is the side of the tongue. Could you please clarify how the rule applies here?

Jazakallah Khairan

Answer

Wa alaikum assalaam wa rahmatullahi wa barakatuh,
The of the definite article in Arabic merges into fourteen letters of the Arabic alphabet, 13 of these and the are , which means "the two close to each other".  The definition of is described on page 30 in the book Tajweed Rules of the Qur'an book Part 2.  It is: The two letters which are close in articulation point and characteristics or close in articulation point but not in characteristic, or close in characteristics but not articulation point. 
The item to make note of is the word "close".  Close is not the same as equal or the same, and as you can see by the definition, some of the letters of the shamsiyyah group are not close in articulation point, but are close in characteristics.  None of the letters of the shamsiyyah group have the same articulation point as the , except the letter itself. 
This is mentioned on page 40 of the Tajweed Rules book Part two, since if two letters have the same articulatioin point, they are not , but instead are either (the same exact letter) or similar, which means they have the same articulatioin point, but do not share all the same characteristics.

In summary, none of the letters of shamsiyyah group have the same articulation point of the except the itself.  They are either close in articulation point and characteristics, or close in articulation point and not characteristics, or close in characterstics and not close in articulation.