Ready-made reading glasses are one-size-fits-all. They have the same power in each eye. Most commonly, we do not have the same prescription in both eyes for best clarity.However,they are great for a back up when travelling, or kept in the glove box of the car. They are great in the garage or hobby workshop where a good pair may get ruined by grease and dirt. They are better economically if you lose your glasses every second week!They are cheap and cheerful for what they can do. But they are best used only for very short periods to avoid eyestrain and headaches.