What does this phrase mean: I walk the line.

[Andycat]

I need some help regarding the meaning of two phrases which occurred in the lyrics of two evergreen songs. The first one is 'I walk the line' and the other one is 'number nine coal'. Thank you.

[Yankee]

I'd say 'I walk the line' means 'I do what is expected' or 'I do what I'm asked to do'.

What song did you find 'number nine coal' in?
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[Andycat]

Thank you very much for the help Amy, my second problem can be found in the song titled Sixteen Tons and it is in the third verse. Look forward to the explanation. ND

[Yankee]

Hi Andycat

That seems to be a reference to a particular type or grade of coal, but I don't know enough about coal or coal mining to say more than that.
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[Johnny_Conrry]

or maybe its a section number coal mine ,for the the lyrics go like this:
"I was born one mornin' when the sun didn't shine
Picked up a shovel and I walked to the mine
I hauled Sixteen Tons of number 9 coal
And the straw-boss said, "Well, bless my soul"

i think it will be usefull to allocate enogh space for country songs exceprtions

[Molly]

"The coal used in this study was West Kentucky number nine coal with particle size ranging from 90 to 30..."

http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5722146