Two circles or three
Two circles give you three regions: A only, B only, and the overlap. That is the right shape for a straight comparison — this option versus that option, before versus after, what we promised versus what we shipped — and it is much easier to read than people expect, because there is only one intersection to argue about.
Three circles give you seven regions, and the centre is where the interesting claim usually lives. Three-circle Venns are the standard form for 'the thing we should do sits where these three considerations meet' arguments: desirable, feasible and viable; what you love, what you are good at and what people will pay for. The risk is that the centre becomes a slogan nobody has tested — if you can fill the centre but not the six regions around it, you have made a poster rather than an analysis.
Beyond three, ordinary circles stop working: four sets cannot be drawn with circles such that every combination gets its own region. If you need four or more, a matrix or a table will serve you better.
Getting the overlaps right
Fill the outer regions first. It is tempting to start with the intersection because that is where the point is, but the exclusive regions are what give the intersection meaning — if everything you can think of lands in the overlap, your two sets are not actually different, and the diagram is telling you so.
Be strict about membership. An item belongs in the overlap only if it genuinely satisfies both labels, not if it is vaguely related to both. Sloppy placement produces the classic bad Venn: a centre full of aspirational words and two nearly empty crescents. When you find yourself hesitating, the usual fix is that a label is too broad — sharpen the label rather than fudging the item.
Watch for the case where a region should be empty and say so. 'Nothing we do sits in both' is a finding worth putting in a document, and it is the kind of conclusion a bulleted list would never have surfaced.
Where Venns are used
In teaching, they are the standard device for compare-and-contrast: two texts, two organisms, two historical periods, two solution approaches. Students fill the overlap last, which is the reverse of the order most adults use and probably the better habit.
In strategy and product work, three-circle Venns frame trade-offs — user need, business goal and technical constraint — and are useful precisely because they force each consideration to have its own space rather than being averaged into a single argument. In logic and set theory, where the notation started, the regions have exact meaning; in a strategy deck they are a visual metaphor. Both uses are legitimate, but it is worth knowing which one you are doing before someone treats your metaphor as a proof.