Meem Sakinah (ميم ساكنة) is the letter meem (م) with a sukoon on it. When Meem Sakinah appears in the Quran, three rules govern how it is pronounced depending on the letter that follows it: Ikhfaa Shafawi, Idghaam Shafawi (also called Idghaam Mithlain Sagheer), and Ith-haar Shafawi. The word "Shafawi" (شفوي) means "labial" or "related to the lips," since the meem is a lip letter.
Ikhfaa Shafawi (إخفاء شفوي) occurs when Meem Sakinah is followed by the letter ب. In this case, the meem is hidden (not fully pronounced), the lips remain slightly pressed together, and a ghunnah (nasal sound) is maintained for approximately two counts. This rule only applies with one letter, making it easy to remember. An example from the Quran is: ترميهم بحجارة.
Idghaam Shafawi (إدغام شفوي) applies when Meem Sakinah is followed by another م. The two meems merge together, and the merged meem is pronounced with a ghunnah of two counts. This is essentially the merging of two identical letters. An example is: لهم مّا يشاءون.
Ith-haar Shafawi (إظهار شفوي) applies when Meem Sakinah is followed by any letter other than ب or م — that is, any of the remaining 26 letters. The meem is pronounced clearly and distinctly without any nasalization or merging. Special care should be taken when the following letter is و or ف, as their proximity to the meem's articulation point might tempt the reciter to hide the meem.