Whoops, apologies! I was posting while very tired, and forgot to preview.
though I recently knitted a 6-foot long scarf; so, there, is the 'long' a redundancy within the rule? Because I probably wouldn't say "I knitted a 6-foot scarf" OR "I knitted a 6-feet long scarf".
I don't think it's a redundancy, just an intensifier; really you knitted a "6-foot-long scarf", so the long functions as part of the measurement that's modifying the noun. I think even if "long scarf" was one concept (as opposed to a short scarf), it would still work, as that would be a single noun compound modified by the measurement.
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though I recently knitted a 6-foot long scarf; so, there, is the 'long' a redundancy within the rule? Because I probably wouldn't say "I knitted a 6-foot scarf" OR "I knitted a 6-feet long scarf".
I don't think it's a redundancy, just an intensifier; really you knitted a "6-foot-long scarf", so the long functions as part of the measurement that's modifying the noun. I think even if "long scarf" was one concept (as opposed to a short scarf), it would still work, as that would be a single noun compound modified by the measurement.