Is it allowed to use a Scrabble dictionary during your turn?

Using a Scrabble dictionary during play seems to me to take away from the game because it is not a word in your vocabulary.

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I agree with you. No, it is not permitted to consult a Scrabble dictionary during your turn.

The dictionary should only be used to adjudicate on challenges (i.e. when a player disputes the validity of a word played by their opponent), and even then it should be done by a third party if possible.

If you don't have a third person around to do the checking, you can prevent a player from 'dishonest browsing' by using the official online Scrabble dictionary to do your lookups.

That's the official take, anyway. But as I often say, if you're playing a game of Scrabble against your sister on the kitchen table, you can use any rule you like ;-)