Thanks for this discussion everyone...I have often assumed (especially while riding) that there is "duplication" among the different chainring and rear-cog combinations. Being a novice bike mechanic, and never being able to get all "24" (3x8) gearings to work without FD rubbing, I adjusted for (and settled for) being able to get (cleanly) the gearings I most wanted (predominantly the "highest" 5 and "lowest" 5). I'm sure my bike gearing could be "dialed in" better than it is now, but it is encouraging to read that some gear combinations should not be used and, therefore, there is no need for those gearings (Big-Big, Small-Small) to be cleanly accommodated by the FD.