SZIR.-Over a year or so ago, you published an article by a priest referring to the use of the safe period, He advised Catholic parents to read a book " Marriage and Rhythm " by Fr. John L. Thomas. S.J., a distinguished American sociologist. available at any Catholic Truth Society bookshop. In it the chapter and chart outlining the Ogino-Knaus safe period shows very simply how any married woman can work out her own safe period. The chapter also gives excellent advice on reasons when the safe period would be unreliable.
I can recommend this book highly.
A Catholic Mother