US Savings Bonds issued by the Treasury are a debt security issued by our government as a way to borrow money from their citizens. Since the mid-1930s bonds have been issued by our government in alphabetical order. Each Bond Series that has come out has been slightly different then the last for a variety of reasons and the all have different rules from the treasury. Each bond since the A series has had a different interest rate and length of maturity. Some are sold at face value, some double in their value as per the terms of their full maturity.
The bond rules have been altered because of trial on error, the state of the current economy and even how the proceeds of the debt security are going to be used.
For example, in 1917 the US Treasury launched the Liberty Bond as a way to generate a cash flow to help fund the expenses of WWI. The sale of the bonds generated $21.5 billion courtesy of the Liberty bonds sales. All the bonds matured at the same time and the US Treasury was not able to redeem all of them for their value. Instead what they did was refinance with debt with variable short and medium-term maturities. Because of this, the government changed the rules on Treasury Bonds to prevent this type of rush from happening again of the future.
In general, the Treasury changes rules on bonds because the times change. What the Series E Bond (a.k.a. defense bond) offered both the investor and the government in 1941 does not match the needs of the modern days.
