Will that be one blade, or a set?

Hi I need a blade for my tractor.  Here's the book.
All right, this is a 42-inch deck and yes we have a set of blades in stock.
Set?  No, it's just one blade.
OK, but it is better if you replace both.
No, the tractor has just one blade.  I took it off.
Umm.  How long was that blade?
About this long (21 inches).
So you must have two blades to get a 42-inch cut...
Huh?
See this 42-inch deck on the floor model?  There's a blade here, and one on the other side.
The other side?  Hmmm.