Final answer:
The U.S. Senate began with 26 members in 1789, with each of the 13 states electing two senators.
Step-by-step explanation:
In 1789, the United States Senate began with 26 members. This was a result of the Great Compromise of 1787 which established a bicameral legislature comprising the Senate and the House of Representatives under the new United States Constitution. At that time, there were 13 states, each of which was entitled to elect two senators, making the initial total membership 26 senators. These senators were originally chosen by the state legislatures, a practice that continued until the adoption of the 17th Amendment in 1913, which allowed for the direct election of senators.