In a section of an essay on concrete poetics (taken from Unoriginal Genius) with this somewhat amusing heading, Marjorie Perloff writes:
As William Marx makes clear in the Introduction to Les arrière-gardes au xxe siècle, the concept of the avant-garde is inconceivable without its opposite. In military terms, the rearguard of the army is the part that protects and consolidates the troop movement in question; often the army’s best generals are used for this purpose. When, in other words, an avant-garde movement is no longer a novelty, it is the role of the arrière-garde to complete its mission, to insure its success. The term arrière-garde, then, is synonymous neither with reaction nor with nostalgia for a lost and more desirable artistic era; it is, on the contrary, the “hidden face of modernity” (Marx 6). As Antoine Compagnon puts it in his study of Barthes in the Marx collection, the role of the arrière garde is to save that which is threatened.
This is said to be a "useful corrective" to going conceptions of the avant-garde. And may well be! I wish only to point out its sympathy with a much earlier remark of Lichtenberg's:
Rational free spirits are the light brigade who go on ahead and reconnoitre the ground which the heavy brigade of orthodoxy will eventually occupy. (H36 in Hollingdale's idiosyncratic numbering)
I like this way of putting it a little better, in fact, in particular its pair of correlated contrasts: the light brigade of rational free spirits and the heavy brigade of orthodoxy. Partly because this suggests that if the heavy brigade's occupation of new-to-it territory does have the effect of shoring up the explorations made by the former, that is not its self-known role (and also that there are not two things: the avant-garde position's no longer being a novelty, and the arrière-garde's ensuring the success of the avant-garde, but rather that the loss of novelty just is the occupation of the territory by the arrière-garde); it's just what happens when the plodding orthodox eventually get there, because they represent the orthodoxy. In one way or another, the surprise eventually wears off and one knows what to make of the thitherto new practice.