TITLE: Is $\lhd$ common notation for "is an ideal of"?
QUESTION [9 upvotes]: This question is because of this comment. I would like to know if I should refrain from using $\lhd$ for "is an ideal of" in ring-theoretic questions. Is it common enough, or should I explain what it means? It's standard notation where I study, but I understand it doesn't have to be everywhere.
Also, is there any strong argument for choosing either $\lhd$ or $\triangleleft$? I don't know much about math typesetting, and I've noticed that people can give surprising and convincing arguments for using one symbol and not using another.

REPLY [12 votes]: I swore I saw this notation somewhere and it took me over an hour to find it.
It's in P.M. Cohn's Introduction to ring theory 2000 ed.
On page 12, he says "One often writes $A \lhd R$ to indicate that $A$ is an ideal of $R$.".
I cannot tell which symbol he used for typesetting.