Soul Edge (or Soul Blade as it was later renamed in some countries) is the first game in the Soul Calibur series, SoulCalibur is really Soul Edge II, SoulCalibur II is actually Soul Edge III, and so one. I never really got that, it would be like having one "Tekken" and then calling Tekken 2 onwards something completly different (btw, it's the same company).

I get that both Soul Edge and Soul Calibur are supposed to be important swords in storyline, but for the most part the majority of characters are still after Soul Edge in one way or another, and in all games except SCIV Soul Calibur doesn't put in an appearance until the very end.