Building panels each comprise an upper first element of solid wood fixed to a lower second element of solid wood. The first and the lower second element are of different wood species. The building panels are provided with a mechanical locking system which comprises a locking strip at a first edge of a first building panel. The locking strip is provided with a locking element configured to cooperate with a locking groove at a second edge of a second building panel for horizontal locking of the first and the second building panels when a tension force is applied. The fiber direction of the first and the lower second elements is essentially along the first and the second edges. The lower second element has about the same or higher moisture shrinkage value than the first element. The locking strip comprises material of the lower second element.