( ( ( ( ( ( The millibar ) ( has ( been ( used ( to ( ( measure ( air pressure ) ) ( for ( many years ) ) ) ) ) ) ) ) , ) but ) ( it ( ( is not ) ( an ( SI unit ) ) ) ) ) . )
(ROOT (S (S (NP (DT The) (NN millibar)) (VP (VBZ has) (VP (VBN been) (VP (VBN used) (S (VP (TO to) (VP (VB measure) (NP (NN air) (NN pressure)) (PP (IN for) (NP (JJ many) (NNS years)))))))))) (, ,) (CC but) (S (NP (PRP it)) (VP (VBZ is) (RB not) (NP (DT an) (NNP SI) (NN unit)))) (. .)))
The millibar has been used to measure air pressure for many years, but it is not an SI unit.
(ROOT (S (NP (NNP Millibar)) (VP (VBZ is) (NP (NP (DT the) (NN unit)) (VP (VBN used) (S (VP (TO to) (VP (VB measure) (NP (NN air) (NN pressure)))))))) (. .)))