Charles didn't precisely learn Latin, although he took several Latin classes. It was quite difficult to bother, given that the translations were constantly floating around in the minds of his professors and classmates. There were always more important things with answers he couldn't simply pluck out of someone's mind, so he passed his classes without so much as remembering how to decline amo.
One phrase stayed in his mind, somehow, despite his total disinterest in the subject once he'd finished enough Latin to be considered a proper gentleman.
nisi in bonis amicitiam esse non posse
Erik was not a proper gentleman, and being more concerned with practical languages, he did not read Latin. All the same, Charles had the phrase engraved on a pocket watch, which he then slid across their now-barren chessboard one evening. Erik simply lifted an eyebrow at it and asked, “What's the occasion?”
Charles just shrugged and grinned before he left for his room.
He considered probing Erik's mind to determine what he thought of the gift, but he decided against it.
“In bonis,” he muttered to himself. “In bonis, Charles.”
Years later, Charles received a key by post with nothing but an M written carefully on the envelope. He told no one, because they would think it was a trap of some sort. Perhaps it was, but Charles could never quite deny Erik his traps.
The key belonged to a safe deposit box at a bank a few miles outside of Westchester. Somehow, Charles was not surprised to find the pocket watch sitting inside with one small piece of paper tied to the chain with string.
It had on it a simple translation of the Latin in the same careful script: Friendship cannot exist except between good men.
He held the watch to his ear, perhaps driven by the old hope that Erik was trying to reach out to him. But the watch made no sound. It had stopped ticking long ago.