Does anyone rember how to calculate impedance?

In the old days when amps and receivers had only one set of speaker outputs, if you wanted to to run more than one pair of speakers you either had to run them in parralell or in series. This varied the amplifierload. Say if I were connecting two eight ohm speaker in paralell, what load would the amplifier see? If I connected them in sereis what load would the amplifier see? The person with the correct answer will receive a vintage slide rule.