Orthophosphoric acid is tribasic but phosphorous acid is dibasic. why?

• Orthophosphoric acid is H₃PO₄ & phosphorous acid is H₃PO₃.
• In orthophosphoric acid there are three hydrogen atoms linked to oxygen atoms but in phosphorous acid there are two hydrogen atoms bonded to oxygen atoms while the third hydrogen atom is directly linked to phosphorous. So orthophosphoric acid is tribasic & phosphorous acid is dibasic.