Charlie Brown had a pencil pal. This pencil pal was originally a pen pal, but Charlie Brown had a lot of troubles with his fountain pen and started to use a pencil, occasionally making attempts to pick up the pen habit.
The story line appears over a decade or more. According to Peanuts Wiki, a return letter from the overseas pal has appeared in the strip: “In the letter, the Pen Pal revealed that he or she had read Charlie Brown’s latest letter to his/her class, and that they all agreed he must be a nice person and someone who is pleasant to know. In response to which, Charlie Brown uttered a vigorous “Ha!” to Lucy.”
Over four decades after their original publication, the craft and perception shown in these cartoons still amazes and delights.
Penmanship is a frequent theme in the cartoon, and the characters (and their off-panel teachers and parents) often communicate with one another via notes and letters.
Recommended reading: “Schulz and Peanuts” by David Michaelis.