Short answer. Then every partner is a manager. Under Article 1803, when the manner of management has not been agreed upon, all the partners are considered agents, and whatever any one does alone binds the partnership. But no partner may, without the others' consent, make an important alteration to the partnership's immovable property, even a useful one.
What the law says
All the partners shall be considered agents and whatever any one of them may do alone shall bind the partnership
Civil Code, Article 1803 — No Management Stipulation. Read the full provision →
Everyone is an agent
Article 1803 supplies the rules when the partners never settled how the firm would be managed. Its first rule is sweeping: all the partners shall be considered agents and whatever any one of them may do alone shall bind the partnership. With no designated managers, every partner is a manager. Each can enter contracts, deal with outsiders and carry out ordinary business, and what any one of them does in that capacity binds the whole partnership.
But opposition still triggers a vote
That broad authority is not unlimited. The first rule applies without prejudice to the provisions governing several managers — meaning the same principle seen elsewhere carries over: a partner may act alone while no one objects, but if another partner opposes the act before it is done, the matter is decided by the majority. So 'each partner is an agent' does not mean each can override the others at will. It means each may get on with ordinary business unopposed, while any partner retains the ability to object and force the question to a collective decision.
Immovable property is different
The second rule carves out one category from the free-for-all. None of the partners may, without the consent of the others, make any important alteration in the immovable property of the partnership, even if it may be useful to the partnership. So while a partner can do ordinary business alone, he cannot single-handedly make a major change to the firm's land or buildings — a significant construction, demolition or conversion — even one he believes would help. Real property is treated as too important to be altered on one partner's say-so. Notice that this bars important alterations specifically; routine upkeep and ordinary use are not what the rule is aimed at, and usefulness is no excuse for skipping the others' consent.
And the safety valve
The rule on immovable property has its own release. If the refusal of consent by the other partners is manifestly prejudicial to the interest of the partnership, the court's intervention may be sought. So a partner blocked from a plainly beneficial change is not simply stuck: where the others' refusal is manifestly against the firm's interest, he can ask a court to intervene rather than let obstruction harm the partnership. As always, the standard is high — manifestly prejudicial, not merely a difference of opinion. For a partnership with no management terms, the sensible course is to know that ordinary acts bind the firm, major property changes need everyone, and a court is the recourse when consent is unreasonably withheld.