Tanto monta, monta tanto, Isabel como Fernando, They amount to the same, or "equal opposites in balance" was the motto

Motto

A motto is a phrase meant to formally summarize the general motivation or intention of a social group or organization. A motto may be in any language, but Latin is the most used. The local language is usual in the mottoes of governments...

 of a prenuptial agreement made by the Spanish Catholic Monarchs

Catholic Monarchs

The Catholic Monarchs is the collective title used in history for Queen Isabella I of Castile and King Ferdinand II of Aragon. They were both from the House of Trastámara and were second cousins, being both descended from John I of Castile; they were given a papal dispensation to deal with...

, Isabella I of Castile

Isabella I of Castile

Isabella I was Queen of Castile and León. She and her husband Ferdinand II of Aragon brought stability to both kingdoms that became the basis for the unification of Spain. Later the two laid the foundations for the political unification of Spain under their grandson, Charles V, Holy Roman Emperor...

 and Ferdinand II of Aragon

Ferdinand II of Aragon

Ferdinand the Catholic was King of Aragon , Sicily , Naples , Valencia, Sardinia, and Navarre, Count of Barcelona, jure uxoris King of Castile and then regent of that country also from 1508 to his death, in the name of...

. During their joint reign they did in fact support each other effectively in accordance with their motto of equality.
The Catholic Monarchs' great sword kept in the Real Armeria in Madrid, made in the 15th century, was used during the reign of Ferdinand II of Aragon and Isabelle of Castile and in all solemn court occasions until the 18th century. With this sword, the Catholic Monarchs Ferdinand and Isabella knighted Christopher Columbus on his return from his first voyage to America. In the Throne Hall of the Royal Palace in Barcelona, Columbus was named "Admiral of the Ocean" and "Viceroy of the Indies". This ceremonial sword was used as the symbol of the royal power in all religious and political ceremonies starting with the conquest of Granada and the beginning of Spain as a nation. On its hand guard it bears the inscription "TANTO MONTA - MONTA TANTO" translates "As much as the one is worth, so much is the other."
The motto "Tanto Monta, Monta Tanto" appeared on the Spanish Royal Standard of the Catholic Kings from 1492-1506. The motto refers to both Catholic monarchs: Isabella and Ferdinand and loosely translates to: "As much as the one is worth, so much is the other" meaning that both King and Queen held equal authority. It was seen on the Spanish flags that Christopher Columbus brought to the New World. Later the motto was changed to "Plus Ultra" which is Latin for "More Beyond" referring to Spain and her lands in the Americas.