There are not rules there are conventions. And they are arbitrary in that they are determined arbitrarily through conventions. Consider heavy slang, accents, connotations and of course uertlty bzirrae siht lkie tihs rnamiineg eretliny ilbilliegtne. So mcuh for the rlues of slilenpg!
.noitanalpxe orez etipsed ,siht daer ot elba eb neve thgim uoy