Part II of Chapter II covers the “two hinges,” Morning Prayer and Evening Prayer:
37. “By the venerable tradition of the universal Church, lauds as morning prayer and vespers as evening prayer are the two hinges on which the daily office turns; hence they are to be considered as the chief hours and celebrated as such.” [SC 89a; see also SC 100.]
For example, when parishes are weighing the introduction of the Liturgy of the Hours, these hours should be considered as the prime candidates. When communities that pray two or more of the hours, Morning and Evening Prayer should stand out from the others in some appropriate liturgical way.
This does harken back to the times when human activity was driven by daylight, and the hours of sunrise and sunset had deeper significance.