Vatican post offices and selected collectors’ shops started selling special stamps this week, marking the period between Pope Francis’ death and the election of his successor.

When a Pope dies, the Vatican issues special stamps with the phrase ‘sede vacante’  (empty seat), valid only until a new Pope is elected.

The so-called “sede vacante” stamps include an image used by the Vatican in official documents during periods between popes: two crossed keys but no papal headgear. They will remain valid until a new Holy Father appears at St. Peter’s window.

Until then, they can send letters, postcards and packages. With the election of the new Pope, they lose the postal value, but the collectable value goes up.

The Vatican printed stamps with four face values: 1.25 euros ($1.42), 1.30 euros, 2.45 euros, and 3.20 euros. They carry the words “Citta del Vaticano”—Vatican City—and “Sede Vacante MMXXV”—vacant See 2025.

All bear a reproduction of the same image: three angels holding up a canopy shielding the symbolic papal keys against white clouds.

The last time the Vatican issued stamps for the interregnum between popes was in 2012, when Benedict XVI broke tradition and retired. The issuing of sede vacante stamps dates from 1929.

Cardinals will gather on May 7 to elect Francis’ successor. The length of the secret election is unknown.