In Id Software’s Commander Keen universe, the Standard Galactic Alphabet, or SGA, is a writing system that all sentient beings, except Humans, use. Nearly all the signs and messages scattered about the game environments in this series are written in this alphabet. It is actually nothing more than a simple cipher where each letter is replaced with a different, alien looking one. The player who learns this cipher is rewarded by being able to read the hints and jokes scattered throughout the game. There is also an entire coded message at the end of Episode 5 that reveals some of Mortimer McMire's plans. There are two places in the series where the entire SGA is written out along with its Earthling counterpart. The first is hidden in a secret level in Commander Keen 3: Keen Must Die, and the second is in another secret level in Commander Keen 6: Aliens Ate my Babysitter.
Tom Hall has this to say on the origin of the SGA:
"I created the Standard Galactic alphabet. I was making the tiles and designing the first level, and I thought I'd put an exit sign there. I decided to make it look like funky futuristic or alien letters. Then I thought, "Hey, I could make a whole alphabet and hide little messages in the game." That's how it started. So I started putting "This is neat" by guns, "Die" by dangerous stuff, and "Hi" all over the place. It was fun to add something that would be all over the place and make you want to explore the levels again."
Tom Hall also says:
“Big secret: now it can be revealed! There is actually a filthy word somewhere in Keen 1-3, spelled in the Standard Galactic Alphabet, made of yellow platforms inside a field of red platforms. I won't say where, but the dedicated can find it... :) Hee!”
Great. I'll have to look for that some time.
ASCII table of letters in the Standard Galactic Alphabet:
# marks the lines, while * symbolizes a dot as on a lowercase i or j.
{A} {B} {C} {D} {E} {F}
## # * ##### # * #####
# # ## # # * * *
# ## # ## #
# ## # ## #
## ##### # ### #####
{G} {H} {I} {J} {K} {L} {M}
# ##### # * # # * #
# # # #* #
## ##### * *#* # #
# # # # #* #
# # # * # # #####
{N} {O} {P} {Q} {R} {S}
# # ### * # * * * #
# # # # # ##### #
# # # # # ###
# # # # # #
# # # * ##### * * #
{T} {U} {V} {W} {X}
##### * * # * * #
# ##### # #
# ##### #
# #
* ##### * * #
{Y} {Z} {period}
# # ###
# # ## ##
# # # #
# # # #
# # # # * # *
References:
http://www.classicgaming.com/cc314/oldcc314/sga.htm
http://www.3drealms.com/keenhistory/keenhistory3.html