The sweet spot is typically at the apex of an equilateral triangle formed with the two speakers. Moving even 6 inches from the sweet spot can noticeably change the stereo image and frequency balance — especially in untreated rooms. The wider the sweet spot, the better the speaker's off-axis response.
Coaxial speakers (KEF, Genelec 'The Ones') have a wider sweet spot because the tweeter and woofer are at the same point in space. Waveguide-loaded tweeters also widen the sweet spot. Room correction can help extend the effective sweet spot by correcting frequency response across a wider area.