Elvis said in many interviews that he tuned his voice everyday. His high quality vibrato shows that he practiced that also when he tuned his voice. Everyday he would play the piano while he sang songs.
His voice was a combination of his large range with the unusual ability to move seamlessly between his tenor and baritone voices.
Elvis Presley’s three-octave vocal range was exceptional, ‘very narrowly all at once a tenor, baritone, and bass’. A 1987 article in the Village Voice included an assessment of his voice in classical terms, categorizing it as a ‘lyric baritone … [but with] unexpectedly rich low [notes] … and astounding high notes’.