We playtested various rules. Some groups like the rules one way. Some like the rules another way. You should experiment to find out what your group likes.
Some clues are invalid because they violate the spirit of the game.
Your clue must be about the meaning of the words. You can't use your clue to talk about the letters in a word or its position on the table. Gland is not a valid clue for ENGLAND. You can't tie BUG, BED, and BOW together with a clue like b: 3 nor with a clue like three: 3. However …
Letters and numbers are valid clues, as long as they refer to meanings. You can use X: 1 as a clue for RAY. You can use eight: 3 as a clue for BALL, FIGURE, and OCTOPUS.
The number you say after your clue can't be used as a clue. Citrus: 8 is not a valid clue for LEMON and OCTOPUS.
You must play in English. A foreign word is allowed only if the players in your group would use it in an English sentence. For example, you can't use Apfel as a clue for APPLE and BERLIN, but you can use strudel.
You can't say any form of a visible word on the table. Until BREAK is covered up by a card, you can't say break, broken, breakage, or breakdown.
You can't say part of a compound word on the table. Until HORSESHOE is covered up, you can't say horse, shoe, unhorsed, or snowshoe.
Homonyms and spelling
English has a lot of homonyms. For example, night sounds like knight, but these two words don't mean the same thing.
Same-sounding words with different meanings and different spellings are considered different words. So you can't give knight-related clues for NIGHT.
Words that are spelled the same are considered the same even though they might have different pronunciations and meanings. For example, actors take a bow and the bow is part of a ship, so you could use bow as a clue for THEATER and SHIP. You could also use it as a clue for archery-related things, even though that bow is pronounced differently.
You are allowed to spell out your clue. For example, if you want your teammates to guess THEATER and STRING, you can spell out b-o-w without committing to a pronunciation. You can give the clue k-n-i-g-h-t even when NIGHT is one of the codenames on the table. (But you can't use t-h-e-a-t-r-e when THEATER is on the table. Theatre and theater are different forms of the same word).
You should spell out your clue if someone asks. If you aren't that strong on spelling, ask the opposing spymaster for help.
Tip: Spelling is not just for homonyms. It's also useful when the room is noisy or when the players have very different accents.
Don't be too strict
England and island were originally compound words, but in this century, island is a valid clue for ENGLAND. Even land is a valid clue for ENGLAND. And anybody who says you can't say sparrow when ROW is on the table is just trying to cause trouble.
If the opposing spymaster allows it, the clue is valid. If you aren't sure, ask your opponent. (Quietly, so the others can't hear).
Sometimes you have to make judgment calls about what is valid and what is not. Different groups may prefer to play the game differently.
English has three ways to write a compound word. Greenhouse is one word. Pack rat is two words. Mother-in-law is hyphenated. Technically, only greenhouse can be a one-word clue.
You can decide to allow any compound words. However, in no case should a player be allowed to invent compound words. Lunar squid is not a valid clue for MOON and OCTOPUS.
Proper names are always valid clues if they follow the other rules. George is a valid clue, but you might want to specify whether you mean George Washington or George W. Bush. Your group can agree to count proper names as one word. This would also allow titles such as The Three Musketeers. Even if you don't allow multiword proper names, you might want to make an exception for place names like New York.
Spymasters should not be allowed to make up names, not even names that turn out to be real. Sue Mee is not a valid clue for CHINA and LAWYER.
Acronyms and Abbreviations
Technically, CIA is not one word. But it is a great clue. You can decide to allow common abbreviations like UK, lol, and PhD. And words like laser, radar, and sonar are always allowed, even though they originated as acronyms.
Some people prefer to allow a more liberal use of homonyms. You can allow knight to be a clue for night-related things if that makes the game more fun for you.
Rhymes are always valid when they refer to meanings. Snail is a valid clue for MAIL because this rhyme is a common phrase. Snail is also a valid clue for WHALE because they are both animals. Snail is not a valid clue for SCALE because their main association is through the sound of the words. (If someone in your group has a job weighing snails, however, this clue is perfectly fine.)
Some people like to allow any kind of rhyming clue. If you decide to allow this, just remember that you aren't allowed to indicate you are giving a rhyming clue. Your operatives will have to figure that out for themselves.
Expert clue: zero
You are allowed to use 0 as the number part of your clue. For example, feathers: 0 means, "None of our words relate to feathers".
If 0 is the number, the usual limit on guesses does not apply. Field operatives can guess as many words as they want. They still must guess at least one word.
If you're not sure why this is useful, don't worry. You'll figure it out.
Expert clue: unlimited
Sometimes you may have multiple unguessed words related to clues from the previous rounds. If you want your team to guess more than one of them, you may say unlimited instead of a number. For example feathers: unlimited.
The disadvantage is that the field operatives do not know how many words are related to the new clue. The advantage is that they may guess as many words as they want.
Penalty for invalid clues
If a spymaster gives an invalid clue, the team's turn ends immediately. As an additional penalty, the other team's spymaster may cover one of his or her words with an agent card before giving the next clue.
But if no one notices that a clue is invalid, it counts as valid.
Keep a straight face
The spymaster is expected to keep a straight face. Do not reach for any card while your teammates are considering the words. When a teammate touches a word, consult the key card and cover the word with the card of the corresponding color. When a teammate chooses a word of the correct color, you should act as though it was exactly the word you meant, even if it wasn't.
If you are a field operative, you should focus on the table when you are making your guesses. Do not make eye contact with the spymaster while you are guessing. This will help you avoid nonverbal cues.
When your information is strictly limited to what can be conveyed with one word and one number, you are playing in the spirit of the game.