When a child spells "stuff" as "stuf", what rule is likely not understood?

Choosing the Floss Rule as the answer reflects a specific understanding of phonetic patterns that govern the doubling of consonants. The Floss Rule articulates that certain consonants are doubled in certain contexts, particularly when a one-syllable word ends with a short vowel and one of the consonants is f, s, or z.

In the case of the word "stuff," the child spelled it as "stuf," which indicates a misunderstanding of this rule because "stuff" is a one-syllable word with a short vowel sound, and the final consonant 'f' should indeed be doubled to conform to the Floss Rule. Therefore, recognizing that the child is not adhering to this rule is crucial in understanding their spelling error. When one partakes in effective reading instruction, it's important to focus on such phonetic rules to aid in correct spelling and decoding.