Mayan writings and graffiti

I'm currently researching the Mayan's written language, and came across this quote from the excellent PBS documentary, Cracking the Mayan Code:

"Each time a Maya scribe wrote a given word or phrase, he could choose from a variety of signs and combine them in new ways. A sign could be written abstractly, or as the head of a human, animal or god. One sign could be slipped inside another, or partly overlapped. Two signs could join, merging their attributes. Glyphs even included figures of monsters and gods."