Short answer: States have constitutions as well. The Constitution is the overall guiding document for the Federal government; state constitutions are foundational/guiding documents for the components that make up the United States.

A good way to think about the US is as a more federalized EU, after several centuries and a civil war to figure out how power devolves between the Federal and State governments. The individual states were originally sovereign entities, bound together through a very weak system called the Articles of Confederation. After this was shown to not work very well, a group of legislators committed treason against the government outlined in the Articles, and convened to write the US Constitution, which the sovereign states decide to adopt after furious debate.

All the states also have their own constitutions.

This is the Texas one: https://tlc.texas.gov/docs/legref/TxConst.pdf

Ah, thank you! And so, what happened in Texas that "made an income tax unconstitutional"?

If you want a bit of reading: https://www.dallasnews.com/news/politics/2019/11/06/early-te...

In Washington's case, the Washington supreme court considered income tax inconstitutional as it would be double taxation as you were already taxed on property.

It's worth noting that they didn't have a state income tax before either. People who live in Texas pay Federal Income Tax, but not State Income Tax.

Texans pay hefty property taxes.

They passed an amendment to their constitution.

Typically the States each also have their own State Constitution. In this case, Texas could have amended their own State Constitution to say something akin to 'The State of Texas shall not impose a tax on the incomes of its residents'.

Each state in the US has its own constitution that defines its governance.