I always thought of Zep and the Beatles as a kind of 'yin and yang'. The Beatles are perceived as very happy and upbeat, but could also be dark and acerbic, and Zep was obviously heavier and darker at times, but had plenty of upbeat, optimistic stuff as well. Also notice how the Beatles songs got longer, jammier, bluesier on the last few albums, kind of pointing towards FM Radio of the 70's. Kinda feels like they were 'passing the torch', greatest band of the 60's to the greatest band of the 70's.
In Chinese philosophy, yin and yang, which are often shortened to "yin-yang" or "yin yang", are concepts used to describe how apparently opposite or contrary forces are actually complementary,[Note 1] interconnected and interdependent in the natural world, and how they give rise to each other as they interrelate to one another.
Yin and yang can be thought of as complementary (rather than opposing) forces that interact to form a dynamic system in which the whole is greater than the assembled parts.