The prevailing style from the 1890s to the 1930s. Old European cut diamonds are the grandfathers of the modern round brilliant cut. With the transition cut being the bridge between the two.
The old european cut has a round girdle, meaning it appears round from the top view as opposed to the earlier old mine cut which had a more squared off top view. Like the old mine cut, the old european has a high crown, small table, and open culet. Notice the culet in the diagram below. Instead of coming to a perfect point, it is cut straight across so that it is flat at the bottom.
Old European cut diamonds have facets that are thick triangular blocks as opposed to modern round brilliant cuts which have thinner facets.