This 1972 song is known for and recognisable by its central theme, a four-note "blues scale" melody harmonised in parallel fourths. The riff, played on a Fender Stratocaster electric guitar by Ritchie Blackmore, is later joined by hi-hat and distorted organ, then drums, then electric bass parts before the start of Ian Gillan's vocal:
"We all came out to Montreux, on the Lake Geneva shoreline
To make records with a mobile, we didn't have much time"
Jon Lord doubles the guitar part on a Hammond B3 organ played through a distorted Marshall amp, creating a tone very similar to that of the guitar. Blackmore uses two fingers to pluck so the pairs of notes can be played exactly simultaneously, to match the organ's timing more closely.
(text from Wikipedia, read the whole article)