There are multiple ways to organize the development of number sense skills. Two of main ones are the Learning Trajectories from Clements and Sarama and the Phases of Development from the Western Australia Ministry of Education. Both approaches layout skills in a clear progression that is linked to a student’s age while the Learning Trajectories lay out the skills in a more detailed manner, the Phases of Development cover a broader age range. Both sets of progressions have been included as they were key in creating the Number Agility levels.