This is an incredibly helpful tool for settling on the layout and arrangement of your intended subject matter before you start painting.

The Tom Lynch Secret Plein Air Tool™ includes:
  
Plexiglas sheath from which pivot a clear plastic sheet with grid and gray scale, a white background sheet, and a transparent red sheet to determine value relationships
  
Dry erase pocket pen
 
The Tom Lynch Secret Plein Air Tool™ is especially helpful with painting cityscapes and achieving consistency with angles and proportions in architecture. 
Tom Lynch also uses it to play with horizons, adding extra trees, fences or buildings to his subject matter before he begins painting.
 
Dimensions: 5" x 3.25" x 0.25" (and dry erase marker is less than 5.5"  long)