Why can twill be woven denser than plain weave?

Because it binds less often. Every interlacing forces a gap between threads; twill has fewer interlacings per area, so the yarns pack closer together, making a heavier, more wind- and water-resistant cloth from the same yarn.

Is twill stronger than plain weave?

Generally yes for tearing and everyday wear, because the packed yarns share load and flex. But plain weave resists snagging better, since it has no long floats to catch and pull.

Side by side

Twill vs Plain Weave

Plain weave and twill are the two fundamental structures of woven cloth. Plain weave interlaces every thread at every intersection, over one and under one, giving a flat, firm, matte cloth. Twill lets each thread float over two or more before binding, stepping sideways each row, which lines the interlacings up into a diagonal and changes nearly every property of the cloth.

Twill

No. 018

2/2 twill · first documented Antiquity

Plain Weave

No. 017

plain weave · first documented Antiquity

The differences

Aspect	Twill	Plain Weave
Interlacing	Over two or more, stepping each row: fewer bindings, longer floats.	Over one, under one: the maximum number of bindings.
Surface	Diagonal wale, slight sheen from the floats.	Flat and even, no directionality.
Drape and density	Denser and heavier for the same yarn, with softer drape.	Firmer and crisper; resists shifting but creases sharply.
Wear	Hides soil and wrinkles in the diagonal; abrades at the float crowns.	Shows wrinkles plainly; wears evenly since every thread is bound.
Classic cloths	Denim, gabardine, serge, tweed, chino, herringbone.	Muslin, poplin, chambray, canvas, gingham, taffeta.

Which to choose

Choose plain weave for crispness, lightness, and stability: shirtings and sheers live there. Choose twill when you want weight, drape, durability, and a surface that forgives wear, which is why almost all workwear and most suiting is twill.

Common questions

Why can twill be woven denser than plain weave?: Because it binds less often. Every interlacing forces a gap between threads; twill has fewer interlacings per area, so the yarns pack closer together, making a heavier, more wind- and water-resistant cloth from the same yarn.
Is twill stronger than plain weave?: Generally yes for tearing and everyday wear, because the packed yarns share load and flex. But plain weave resists snagging better, since it has no long floats to catch and pull.

Full entry: Twill Full entry: Plain Weave

Sources & References

1.Twill, Wikipedia
2.Twill, Encyclopaedia Britannica
3.Plain weave, Wikipedia
4.Plain weave, Encyclopaedia Britannica