How are the next puzzles chosen for me when I am training?

By default, your next puzzle is randomly selected from a pool of puzzles with ratings close to your own. As your rating increases or decreases, the pool of puzzles available to you changes accordingly.

If you'd rather focus on a specific type of puzzle, you can use Custom Puzzles to practice unrated puzzles based on the themes you choose. To get there, hover over Puzzles in the left sidebar of any Chess.com page and select 'Custom Puzzles.'

From there, check the box next to each theme you'd like to practice. You can select just one theme or choose multiple themes at once!