Surah 55. Ar-Rahman, Ayah 17

I.e., of the extreme points of sunrise and sunset in summer and in winter (see 37:5 and 70:40 ), including "all that is between them": a metonym for God's being the Ultimate Cause of the orbital movement within the universe.

 The two points of sunrise and sunset in the summer and the winter.

The two Easts are the two extreme points where the sun rises during the year, and includes all the points between. Similarly the two Wests include the two extreme points of the sun's setting and all the points between. The Dual Number fits with the general atmosphere of Duality in this Sura. Allah is Lord of every region of the earth and sky, and He scatters His bounties everywhere. See also n. 4641 to xliii. 38, and n. 4034 to xxxvii. 5.