Whereby and hereby, in particular, are very formal and rarely used in day to day speech or writing. Whereby - by which "a system whereby people could vote by telephone" You use hereby when formally confirming what you are doing. I hereby sentence you for life after all the charges against you have been proven true. Thereby means "by that means", or "as a result of that". The government increased spending on schools, thereby ensuring that all children could receive a good education.