Not all senators were equal. There was an order of precedence within the Senate based on the highest office to which individual senators had been elected (and, within those groups, on the judgment of the presiding consul). In addition to the official gradations; there was a group of lesser senators colloquially called pedari, "footmen". Even the Romans debated exactly what this meant. SOme thought it alluded to a right purportedly won by men who had held the higher offices, to come to the Senate in a chariot. The most popular theory today is that the pedarii were far enough down the line of seniority that they voted but were rarely called on to give their opinions. Voting, was conducted by gathering on different sides of the Senate house. Thus pedarii only spoke with their feet.

Source: John C.Rolfe, trans. The Altic...