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The Floss Rule: What You Need to Know

One of the principles of Structured Literacy is explicit, direct instruction – which means not leaving it up to chance that students will figure out patterns within print by themselves. To know which patterns and rules to teach our students, however, we must have a solid understanding of our own. In this week’s post, we are digging into the Floss Rule: What it is AND how to teach it!

A student desk with letters and worksheets to practice the floss rule


This spelling rule dictates when the sounds /f/, /l/, and /s/ are spelled with doubled letters –ff, -ll, and –ss at the end of words. It is called the “FLoSS” rule as a way to remember the three letter-sound correspondences to which it relates. The rule is as follows:

A textbox stating to double the final f, l, or s when a short vowel is immediately followed by final sounds /f/, /l/, or /s/ in a one syllable base word

The rule is also sometimes presented as a checklist, like the one below:

If a word has a…

  • One syllable base word

  • Final /f/ /l/ or /s/ sound

  • Final /f/ /l/ or /s/ sound immediately following a short vowel

Double the final f, l, or s.

A key takeaway for students when learning the Floss Rule is that the two final letters represent just one sound. Knowing the specific conditions for when these letters double is necessary for spelling.

“Guardian” Consonants

If you look across multiple spelling rules (e.g. Floss, -ck, -dge, -tch) there is a clear pattern: syllables with short vowels attract “protection” of extra consonants. In the Floss Rule, the final consonant is doubled when immediately following a short vowel. For the final /k/, /j/, and /ch/ sounds, those extra consonants are also added when it follows a short vowel. As Louisa Moats explains in “Speech to Print” (2020), “Extra consonants wrap a vowel inside a closed syllable, and prevent it from bursting forth as its name.” You also can see this pattern when it comes to doubling rules for suffixes, as in swimming or clapped. Drawing connections like this for your students can help demystify the English language – maybe it’s not quite as random as it’s often made out to be!

To “Z” or Not to “Z” ?

In some curriculums, doubled final “z” is included in this rule as well. It explains words such as jazz, fizz, fuzz, and buzz. However, many other programs do not teach the /z/ sound as part of the Floss Rule, because there are many more exceptions to this sound than the others. It is further complicated by the letter “s” representing the /z/ sound at the end of many high-frequency words. Consider these exceptions to a doubled final “z”: quiz, whiz, his, as, is, has, was.

Expect the Exceptions!

Even without final /z/, there are always some exceptions to spelling rules. Other exceptions to the Floss Rule include, but are not limited to: if, yes, plus, this, pal. Some exceptions, like bus and gas, can be explained by the fact they are shortened versions of the words autobus and gasoline. Others can be explained through etymology. For example, if, yes, and this are all derived from Old English. Old English words make up many of the truly irregular words in the English language.


Vocabulary and Background Knowledge

It is important for students to know certain vocabulary to understand the Floss Rule. Students should know which letters are vowels and consonants, and be able to accurately identify the vowel within a word. Without this background knowledge, the rule and/or checklist will have little meaning.

Recognizing the term “short vowel,” as well as an ability to distinguish between short and long vowel sounds, can support understanding too – especially if older students are being introduced to this rule. We often do a quick review or sorting activity with long and short vowel sounds prior to starting a lesson on the Floss Rule.

Introduction (Discovery Method)

When introducing this rule to students, we like to start by focusing on the speech sounds. Here is an example of what we might say:

“Listen to these words: hill, bell, doll. What sound was the same in each word? /l/ What position did you hear the /l/ sound? The final position. Watch as I write these words on the board. What do you notice?”

These steps can be repeated with words for -ff and -ss. It’s important to highlight that the two letters represent just one sound, especially if students haven’t been introduced to other digraphs yet.

Decoding Practice

When students are introduced to a new phoneme-grapheme correlation, we want to provide as much opportunity as possible to read that pattern across various contexts. We decode words, followed by phrases, and then sentences as students grow more confident and proficient at reading the grapheme. When they’re ready, we’ll read a longer decodable text that contains a concentration of words following the Floss Rule. We might provide fluency grids, roll and reads, or other skill-based games for students to use as warm-ups, independent practice, or follow-up work at home!

Spelling Practice

While students are often quickly able to master decoding words with this pattern, spelling can be more tricky. Here are some ideas for scaffolded spelling practice!

  • Sorts: There are several variations of sorts that could be used. For example, full words to sort into “Floss Rule” or “Not Floss Rule”. Picture sorts, where students must listen to the final sound in the word and/or the vowel sound and sort into “Floss Rule” or “Not Floss Rule”.

  • Complete the word: Provide students a practice page with words that are missing the final grapheme (e.g. hi__ stu___ shel___ mea__). Dictate to students what the word should be, and they must decide whether or not to apply the Floss Rule.

  • Dictation: Dictate Floss Rule words for students to segment and spell. Add in several non-examples to monitor whether or not students are able to apply the rule.

Keep checking back on our blog for upcoming posts about other spelling patterns soon!

Written by: Rachel Draper & Taryn Quaytman

Copyright © 2023 SparkEd LLC



Moats, Louisa C. (2020) Speech to Print: Language Essentials for Teachers (3rd Edition) MD: Paul H. Brookes Publishing.


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