This study aimed to investigate how the content of the lessons makes it possible for thepupils to discern the sound of the vowel that precedes the double consonant. Researchof spelling shows that spelling with double consonants is one of the most difficultspelling rules in the Swedish language. It is fundemental for pupils to discern the soundof the vowel to be able to decide if there shall be one or two consonants immediate afterthe vowel.With the variation theory as starting point we implemented a learning study with pupilsin the age of 10-12 years old. The aim was to find out what was decisive for the pupils´possibility in discerning the sound of the vowel that precedes the double consonant.Three lessons were implemented and were documented by video. After each lesson weanalised how the teacher handled the lesson contents and then compared the pupils´ability in spelling words with double consonants before and after the lesson.The result from the three lessons showed that the pupils´ ability to discern the sound ofthe vowel which precedes the double consonant were different depending on the useddenomination. During the first lesson the pupils had possibility to discern the sound ofthe vowel in the denomination sounds/does not sound like in the alphabet. During boththe second and the third lesson the denomination long/short vowel were used. Incomparison between the results of the test before and after the lessons, the studyshowed that the pupils from the first lesson proved to have the greatest improvement inthe ability in spelling words with double consonants. A finding what was decisive (thecritical feature) for the pupils´ learning in the study was the denomination of the soundof the vowel.