Can I build a model out of both DUPLO blocks and LEGO blocks? Do they fit together?
Yes! You can add LEGO on top of DUPLO, and DUPLO on top of LEGO. Here's an example picture from The Brick Blogger:
Because stacking LEGO on top of DUPLO requires the particular LEGO tube design on the bottom (to fit in the hollow top of the DUPLO bricks), many "LEGO-compatible" bricks do not share this compatibility with DUPLO - at best, you can stack DUPLO on top of them, but not the other way around.
In addition, you can use only some types of LEGO bricks with DUPLO. Plates, tiles and other modified pieces can't stack with DUPLO. DUPLO can sit happily on top of plates, but plates and tiles only balance on top of Duplo, there's no real clutch between them that way.
Some elements are compatible, others are not. IIRC
- You can stack regular 2x2, 2x4 etc lego bricks on top of normal duplo studs and they fit and hold fine.
- You can't cover only part of a stud, so bricks with odd dimensions can't stack completely on top of duplo and 1xn parts can't stack on top of duplo at all.
- Bricks with curved edges are similarly going to be incompatible.
- Thin plates don't have enough depth to take up the taller Duplo stud.
- You can stack Duplo bricks on Lego studs but only the outside of the brick grips the studs which in my experience leads to a relatively weak connection.
- Duplo base plates have studs with flat tops. These are incompatible with Lego.
It seems that the basic Duplo-compatible Lego bricks are a lot less common than they used to be. I just looked at a "Large Creative Brick Box" and found that only about 16% of the contents was traditional Duplo-compatible bricks. Themed sets (including the "Juniors" range ) often contain hardly any.
Yes, the even by even sized bricks fit to the Duplo studs, can't overhang. Lego plates on top leave a gap. Could overhang from that and add bricks under it, but why bother as it costs more (A 2×2 Duplo brick costs 50p, a 2×4 LEGO System brick costs 19p). Any Lego fits under, but only some Duplo hold with fancy tubes then still not at the corners. The Duplo brick's walls have the same thickness as older normal Lego, with ribs.
In fact older 2×3 without cross support fit if over hanging, so this just leaving 3 normal Lego studs on the corners not over hanging. A 2:3 slope 3×3 fits when over hanging corner, so could build a roof with eaves.