This formula grants the opposition its longstanding demand to secure at least one-third of the cabinet, granting it a veto over any legislation to which it objects.
The government and its March 14 supporters have consistently rejected the opposition's demand for the one-third share. That demand was a major reason for the political gridlock that has bedeviled Lebanon since November 2006, when opposition ministers quit the government, sparking the crisis.
The two sides also agreed on an electoral law to govern next year's parliamentary polls and vowed to begin a dialogue under the new president's auspices to discuss the fate of Hezbollah's arms.
The United States, which considers the Shiite group a terrorist organization, will probably be uncomfortable with its new power within the government.
The Bush administration has strongly backed Mr. Siniora's government and has called repeatedly for the disarming of Hezbollah, which has been rearming in southern Lebanon following the summertime 2006 war with Israel.
"The issue of Hezbollah's arms will remain a sticking point and a major concern for the Bush administration and hence for March 14," Amal Saad-Ghorayeb, an expert on Hezbollah, told Reuters.
"Secondly for Hezbollah, there will continue to be the main problem of foreign allegiances, so Lebanon will remain in this tug of war between the United States and Saudi Arabia on one hand and Iran and Syria on the other," she added.