Three.js / WebGL - transparent planes hiding other planes behind them

Question

When you have two planes in Three.js / WebGL and one or both of them are transparent, sometimes the plane behind will be hidden by the transparent plane above. Why is this?

Answer 1

Let's say that you are using some transparent *.png image. Then this would help:

new THREE.MeshBasicMaterial( { side:THREE.BackSide,map:texture, depthWrite: false, depthTest: false });