The Texas Constitution, like that of the United States and other state and national governments, establishes the fundamental laws under which all of it's citizens are governed. The Constitution that we use today was first adopted in 1876. However, Texas has operated continuously under a constitutional government since 1824--a dozen years before the Fall of the Alamo!
Under the Mexican Constitution of 1824, the regions of Texas and Coahuila were combined into one state. Soon afterwards, the State Constitution of Coahuila and Texas was adopted and governed the region.
Dissatisfied with their union with Coahuila and other government policies, the Texans drafted and proposed a Constitution of 1833 which would give Texas separate statehood within the Mexican Republic. However, this proposal was rejected by the Mexican government.
The Convention which met at Washington on the Brazos and adopted the Texas Declaration of Independence, also framed the Constitution of 1836, which served throughout the period of the Republic of Texas. This was replaced when Texas achieved statehood by the Constitution of 1845, approved by both the people of Texas and the Congress of the United States.
At the outbreak of the Civil War, the Secession Convention met in Austin and modified the Constitution of 1845 to reflect Texas' new alliance with the Confederacy. After the war, this was replaced by the Constitution of 1866, which served during much of the Reconstruction Era following the defeat of the Confederacy. Further changes were implemented in the Constitution of 1869, the most verbose of all Texas constitutions.
Finally, the Constitution of 1876 was adopted soon after the end of Reconstruction in Texas. With its various amendments, this document has now endured for over one hundred and twenty years.