You asked 1) 'What is a 'linear foot'' 2) 'how much wood would you
would get' and 3) 'how much would it cost.'
1) A linear foot is the length of the piece that you are getting. The
reading on your tape measure.
2) As far as 'how much' or the volume of the wood. For a 1 foot long
piece of 2" thick by 12" wide lumber; you would get 2 board feet. A
board foot is a measurement of volume equal to 1 foot of 1"x12" lumber.
By looking at the price per bf you can figure how much your going to
spend for whatever volume of wood that you need.
Using the example above; at $7 per linear foot of 2" x 12" translates
into $3.50 per boardfoot (bf). ($7 / 2bf = $3.50/bf)
Now lets say that you change your project. Using that same cost for bf
($3.50 per bf). For this example project lets say that you need 10 feet
That is 15 bf (10 x (3x6)/12)
Your total cost is $52.50. (15bf x $3.50/bf)
The price per linear foot on that lumber is $5.25 ($52.50 / 10ft)
3) It cost $7 times the reading on your tape measure.
Hope that helps.