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

Setting the depthWrite property to false solved my issue.

new THREE.MeshBasicMaterial({ 
    opacity: 0.25, 
    transparent: true, 
    side: THREE.DoubleSide, 
    depthWrite: false
});