“That’s FGGBD.”
“Fine,” Mason said. “Now, ‘fingerprint.’ That’s page 377, the seventh word on the page.”
Della Street said, “Three-seven-seven-A-seven. That’ll be EIIAI.” Abruptly, she looked down at what she had written and began to laugh.
“What?” Mason asked.
“I was just wondering what would happen if Lieutenant Tragg got hold of this message,” she said. “Has it occurred to you, Chief, that out of four words, two of them have ended in AI?”
Mason frowned, scratched his head. “That isn’t so good,” he said. “It’ll give Tragg too much of a clue. He’ll know darn well then it isn’t just an ordinary cipher, but some sort of a code.”
“You don’t think he’ll get hold of this, do you?”
“He may.”
“I don’t see just what you’re planning to do. Won’t the man who gets the message know it’s a trap?”
“Not if my idea is correct. The persons who are using this means of communication both have access to that place in the cellar; but for some reason, they don’t dare to be seen talking together. Now if that’s the case, they won’t have any opportunity to clarify an ambiguity in the case. In other words, the person who gets the message can’t pick up a telephone and say, ‘Hello, Bill. I got your message. What do you mean, a fingerprint? Your fingerprint or my fingerprint. Or...’ ” Mason broke off suddenly to stare at Della Street. “Do you realize,” he demanded, “what I have just said?”