Overdrive

In terms of gain, overdrive ends where distortion starts. Take your original tone an "Overdrive" it to create a pushed gainey tone. Unike distortion, which changes the sound completely and saturates the signal, an overdrive can keep the core of your clean tone and dirty it up. Use it on a clean amp for some slight grit, for pushing already broken amps up into higher gain or tighten up your heavy tones by stacking OD pedals into distortions.