Instead of using a guitar pick, Montgomery plucked the strings with the fleshy part of his thumb, using downstrokes for single notes and a combination of upstrokes and downstrokes for chords and octaves. Montgomery developed this technique not for technical reasons but for his neighbors. He worked long hours as a machinist before his music career began and practiced late at night. To keep neighbors from complaining, he began playing more quietly by using his thumb. This technique enabled him to get a mellow, expressive tone from his guitar. George Benson, in the liner notes of the Ultimate Wes Montgomery album, wrote, "Wes had a corn on his thumb, which gave his sound that point. He would get one sound for the soft parts, and then that point by using the corn. That's why no one will ever match Wes. And his thumb was double-jointed. He could bend it all the way back to touch his wrist, which he would do to shock people."
In fact there is a good web-site that teaches how to play like Mr Montgomery located at Beginner Guitar HQ (The link opens a new Window)