Not so; indicating that something is different or the opposite of what was stated.
彼の説明は正しいかもしれないが、そうでない点もある。
His explanation may be correct, but there are points that are not so.
こちらの条件に合わない、そうでない場合は別の方法を考えよう。
If this doesn't meet your conditions — if not — let's consider another method.
それが真実だとは限らない、そうでない可能性もある。
That isn't necessarily true; there's also the possibility that it's not so.