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pen

In the southern United States, a unit for a quantity of fuel wood in stick diameters. The sticks were laid to form a square. Each layer consisted of two sticks, with another two sticks laid on top at right angles, and so on like a child's Lincoln Logs, until the pile was 6 feet high.

If the sticks were 4 feet long, 5 such pens were accepted as equal to a standard cord of fuelwood. If the sticks were 5 feet long, 5 pens were equivalent to a long cord (160 cubic feet).

Bertram Husch, Thomas W. Beers and John A Kershaw, Jr.
Forest Mensuration. 4th edition.
John Wiley and Sons, Inc. 2002.

Page 203.