Consider $L^2([0,1],\mathbb{R})$.
Then, $$1, \sqrt{2} \cos(2 \pi j x), \sqrt{2} \sin(2 \pi j x ), \quad j =1,2,\ldots$$
is a Schauder basis on $L^2([0,1], \mathbb{R})$.
I am curious, how does this generalize for the Sobolev space $W^{2,2}([0,1]^2, \mathbb{R}^2)$ ?