The meem saakinah rules, part 2 (idghaam mithlayn sagheer)
Merging ( 
) of the 
saakinah
As a review, the linguistic definition of 
is: merging, insertion.
The applied tajweed definition is: The meeting of a saakinah letter with a voweled letter so that the two letters become one emphasized letter of the second type (of letter).
The of the 
saakinah occurs only with one letter: another . Whenever a saakinah letter comes before a voweled letter of the same articulation point and characteristics, in other words the same exact letter, there is merging of the saakinah letter into the second letter. This merging is called 
, or the small merging of two of the same. The “small” refers to the first letter of the being saakin and the second having a vowel.
If there is a saakinah followed immediately by a with a vowel in the in the same word or between two words, we are then required to insert the saakinah into the with a vowel, which then acquires a shaddah. There is a ghunnah then on the with a shaddah. The only time there is an of a into another in one word is in the case of the individual separate letters that start some surahs. The group of letters is considered one word. In other words, 
is considered one word, so the saakinah at the end of the written out letter 
merges into the voweled which is the first letter of the written out letter . Outside of the individual separate letters at the beginning of some surahs, the of the saakinah into a voweled takes place between two words.
Examples
Click here to listen to the aayah
In the above aayah, there are two examples of merging of the saakinah into a voweled . In the first example, the second word of the aayah ends with a saakinah, and the word immediately following, 
has a voweled as its first letter. The saakinah then merges into the voweled which then acquires a shaddah, and there is an accompanying ghunnah for a period of time that should be learned from a qualified Qur’an teacher. What was said about the first example, applies to the second, both of which are denoted by red letters.
Another example is found in aayah 20 of surah Al-Burooj. Please note the idghaam of the saakinah into the voweled is shown in red letters:
