The number of sedevacantists is largely unknown, with some claiming estimates of tens of thousands to hundreds of thousands. Many active sedevacantists are involved with traditionalist chapels, societies and congregations, such as the Congregation of Mary Immaculate Queen or the Society of Saint Pius V, attending their chapels for Mass and Confession; other sedevacantists attend services of the Eastern Catholic Church or the Society of Saint Pius X, although the SSPX officially condemns sedevacantism. Sedevacantists claim that the post–Vatican II Mass is invalid or illegitimate.