Page and Plant are the targets of a lawsuit brought by the trustees of a member of the band Spirit, who claim Led Zeppelin stole the 1971 song's opening guitar cords from Spirit's 1967 instrumental track "Taurus." The suit asks for just $1, and for Spirit to get credit for that song opening.
Plant's testimony is considered an important element in the defense.
Plant, wearing his long hair back in a pony tail, spoke softly during testimony, saying he and Page sat by a fire 45 years ago in a recording retreat where Page played the opening of what would become one of Led Zeppelin's greatest hits.
He said he was working on a lyrical couplet reminiscent of the culture and mystique of the Welsh countryside, then recounted the lyrics.
"There's a lady who knows all that glitters is gold and she's buying a stairway to heaven," Plant said in testimony. "When she gets there she knows if the stores are all closed, with a word she can get what she came for."
Page testified last week. describing how he worked to create a song building from an acoustic base leading in to Plant's vocals.
Closing arguments take place Wednesday, then the case is handed to the jury.