  Nearby companions alter the evolution of massive stars in binary systems.
Using a sample of Galactic massive stars in nearby young clusters, we
simultaneously measure all intrinsic binary characteristics relevant to
quantify the frequency and nature of binary interactions. We find a large
intrinsic binary fraction, a strong preference for short orbital periods and a
flat distribution for the mass-ratios. Our results do not support the presence
of a significant peak of equal-mass `twin' binaries. As a result of the
measured distributions, we find that over seventy per cent of all massive stars
exchange mass with a companion. Such a rate greatly exceeds previous estimates
and implies that the majority of massive stars have their evolution strongly
affected by interaction with a nearby companion.
