اجْتَمَعَتِ الْوَاوُ وَالْيَاءُ فِي كَلِمَةٍ وَاحِدَةٍ، وَالْأُولَى مِنْهُمَا سَاكِنَةٌ.
Translation: Wāw and yāʾ came together in one word, and the first of them was sākin (without a vowel).
Here the first letter is wāw and the second is yāʾ. The book specifies not only their adjacency but also that the first is sākin as part of its explanation.