Ruy Lopez: 3rd move sidelines - Part 1: Repertoire Preparation
Sidelines are sidelines because they are bad for one of the sides. I realized this while studying the 3rd move alternatives for Ruy Lopez.
First, there is the Steinitz defense. It is bad because of a masterful trap hidden in it - called the Tarrasch trap - in which many great masters have been snared. Note that "Steinitz defense deferred" is a very respectable opening.
Here is the trap
This is why Black is forced to "give up the center" by playin 8...exd4.