( ( this ( time period ) ) ( ( is ( called ( an interval ) ) ) . ) )
(ROOT (S (NP (DT this) (NN time) (NN period)) (VP (VBZ is) (VP (VBN called) (NP (DT an) (NN interval)))) (. .)))
this time period is called an interval.
(ROOT (S (NP (NN Period)) (VP (VBZ is) (NP (NP (DT the) (NN time) (NN interval)) (VP (VBN required) (PP (IN for) (NP (CD one) (JJ complete) (NN wave) (S (VP (TO to) (VP (VB pass) (NP (NP (DT a) (NN point)) (VP (VBN called))))))))))) (. .)))