We investigated the strategies involved in written addition and the factors that influence the choice of strategy. Forty-eight adults were recruited to solve 60 twodigit additions on a graphics tablet. This allowed us to register the position from which the participants wrote the solution down, the initiation and solution times and the correctness. Starting the solution from the units indicates the use of the columnar algorithm, whereas starting from the tens or the hundreds implies the use of a flexible strategy. We found that while anxiety does not influence the choice of strategy, arithmetic skill does. Higher-skilled participants solved the additions more often from the units than lower-skilled ones. The adopted strategy depends furthermore on problem types...