I was instructed to add a comma between get and the below, but I was never told why:
The further towards the right you get, the more liberal they will be.
Why are we taught to do this?
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Neither the head clause 'the further towards the right you get' nor the subordinate clause (the dependent clause) 'the more liberal they will be' can stand alone as sentences; they must be joined together.
But in joining them, quite apart from any grammatical rules that need to be observed for the sake of convention, the verb from the head clause may appear, at first, to take 'the more liberal' as a direct object :
The further towards the right you get the more liberal they will be.
This, at first glance, can be confusing or ambiguous.
But with the addition of the comma, the whole becomes a very balanced sentence and a powerful one, for the head clause leads in to a somewhat unexpected conclusion beyond the pivot point of the comma 'the more liberal they will be', expressing how the political spectrum really works, other than might have been supposed.
Rearranged into a more compact sentence, the statement does not have quite the same impact as it does in the pivotal way it is originally presented - as two contrasted clauses.
There is more on this kind of construction elsewhere on SE where it is made clear that it is a :
comparative correlative or conditional comparative type of construction.