One major difference between the Assembly and the Council of 500?

The assembly was attended by all male citizens of Athens, Councils of 500 was only attended by the Citizen's Representatives.

Explanation:

In the past, Athens held the Assembly around 40 times a year. All male citizens were expected to gathered in one place to discuss various issues such as, taxation, war/security, religion and treaties/alliance with other nations.

Councils of 500 on the other hand, was a attended by representatives that were elected from the pool of citizens. They chose these representatives using a lottery system. (It's based on luck rather than through voting)

debating and finalizing the detail of the policy

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