Two cards, one choice
1At a fictional design exhibition, visitors may choose a blue card or a green card. Both cards give access to the same room in the morning. The colour does not change the display a visitor can see. However, the desk asks each visitor to take only one card. A volunteer says, "Either card is valid." She means that either choice will work, not that a visitor should take the entire pair.
2After lunch, the green cards expire, while the blue cards remain valid. The volunteer removes her earlier notice and writes that the blue card is valid but the green one is not. A colleague proposes using "Neither card is valid" because one type has expired. She rejects his suggestion. It would tell visitors that the blue card has also stopped working. She draws two boxes to show why the proposed sentence changes the facts.
3On the next day, the exhibition room is closed and neither card is valid for that day. This time both boxes receive a cross. The volunteer writes, "Neither of the cards is valid for today." She is describing only these two exhibition cards, not every ticket issued by the building. She also avoids adding another not after neither. Before posting any notice, she checks the status of each card and then checks the grammar. Her colleague learns that the number of possible choices, the status of those choices, and the number a visitor may take are three separate pieces of information.