I think this is all fron Nunan Second Language Teaching & Learning; I know I have p. 12 written in the margin of my notes and my notes correspond to that page.
I like his comment: a unit or lesson is a sequence of tasks; that units or lesons get their coherence from sequencing and integrating .based on a principle.
What is your principle?
A big change occurred in the 70s when communication began to be seen as an integrated process and not a set of discrete learning outcomes.
A big change occurred then on the part of researchers and eventually teachers when they began to be able to know the difference between knowing grammar rules and being able to use the rules in communication.
My own take on this (and I’ll have to reread Nunan; just glancing at it today made me want to read his stuff again) is that Before the 70s there had been no connection between linguists and fl teachers other than the failed audio-lingual method. I remember how difficult it was to find a linguistics course in the 60s but most people graduating now have had some brush with linguistics and fl majors get a fair amount.
It was that growing familiarty with linguistics in the 70s and 80s that allowed fl teachers to see a language as a self-regulating system in Larsen-Freeman’s words.
We see the ignorance of this when fl teachers still decry “incorrect grammar” as leading to chaos and a failure of communication even in the students’ native language. They do not understand language as a system.