( ( A ( regular octahedron ) ) ( ( is ( ( a ( Platonic ( solid composed ) ) ) ( of ( ( ( eight ( equilateral triangles ) ) , ) ( ( four ( of which ) ) ( meet ( at ( each vertex ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (JJ regular) (NN octahedron)) (VP (VBZ is) (NP (NP (DT a) (JJ Platonic) (JJ solid) (NN composed)) (PP (IN of) (NP (NP (CD eight) (JJ equilateral) (NNS triangles)) (, ,) (SBAR (WHNP (CD four) (WHPP (IN of) (WHNP (WDT which)))) (S (VP (VBP meet) (PP (IN at) (NP (DT each) (NN vertex)))))))))) (. .)))
A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex.
(ROOT (S (NP (NP (DT The) (CD eight) (NNS sides)) (PP (IN of) (NP (DT an) (NN octahedra)))) (VP (VBP are) (PP (IN in) (NP (NP (DT the) (NN shape)) (PP (IN of) (NP (DT a) (NNS triangles)))))) (. .)))