It is believed that the apostle Andrew was crucified on a saltire (X-shaped) cross; hence, the name St. Andrew's Cross. He is supposed to have told his executioners that he was not worthy to be crucified on the same cross style as Jesus, and persuaded them to alter the shape. Detailed records of his crucifixion only date back to the Middle Ages, and these records are influenced by the imagination of the medieval artists. This cross shape reminds Christians that they should exercise humility.
Before Peter was crucified, he too requested that a cross different to Christ's Latin Cross be used. Therefore, we have another cross that Christians associate with humility, which is the upside-down Latin cross