What are the verses of tranquility (ayat al-sakinah), and why, and in what context are they recited?

Tranquility (sakinah) is a serenity that Allah casts into the hearts of His servants to strengthen them in times of fear. Allah has mentioned it in six places in the Holy Quran, and it is a gift, not something earned. Sheikh al-Islam Ibn Taymiyyah, whenever affairs became difficult for him, would recite the verses of tranquility. Ibn al-Qayyim did likewise, and they both found it to have a great effect on the heart's peace and serenity. The essence of tranquility (sakinah) is the calmness, dignity, and stillness that Allah bestows upon the heart of His servant when it is agitated by intense fears, thereby increasing their faith, certainty, and steadfastness. Allah sent it down upon His Messenger and the believers in situations of anxiety and turmoil, such as during the Hijrah and the battles of Hunayn and al-Hudaybiyyah. It is permissible to recite the verses of tranquility in situations of fear, tribulations, distress, and whispers, in the hope that Allah will strengthen the heart, without believing that its recitation is a recommended act of worship in a legislative sense.