Why does a test garment get called a muslin?

Because dressmakers traditionally cut the trial version of a garment in cheap unbleached cotton. America kept the cloth's name for the practice, while Britain calls the same trial garment a toile and the cloth calico.

What made historical Dhaka muslin special?

Hand-spun yarns of almost impossible fineness, woven into cloth so light it earned names like woven air. The handloom trade collapsed under industrial competition in the nineteenth century, and the modern word drifted to mean ordinary cheap cotton.

Side by side

Muslin vs Calico

Muslin and calico are both plain-weave cottons named for the trading ports that shipped them, Mosul and Calicut, and both words mean different things in Britain and America. Generally, muslin is the finer and lighter cloth and calico the coarser, but in US sewing rooms the cheap fitting fabric is called muslin while in British ones the same cloth is called calico.

Muslin

No. 027

plain weave · first documented Antiquity

Calico

No. 028

plain weave · first documented 1100s

The differences

Aspect	Muslin	Calico
Fineness	Fine to gauzy; the historical Dhaka muslins were the finest cottons ever woven.	Coarse and sturdy, often with visible husk flecks in the grey cloth.
Name origin	Mosul, the trade city through which Europe met the cloth; the great weaving center was Bengal.	Calicut (Kozhikode) on India's Malabar coast.
US usage	The unbleached test-garment cloth in every sewing room.	Small all-over floral prints, as in a calico dress or calico cat.
UK usage	A fine cloth, also the gauze of kitchens and dressmaking.	The plain unbleached cotton used for toiles and bags.

Which to choose

If you are buying fitting fabric, ask for muslin in America and calico in Britain; you will get the same cloth. Used precisely, muslin should name the finer, more open weave and calico the heavier utility cotton.

Common questions

Why does a test garment get called a muslin?: Because dressmakers traditionally cut the trial version of a garment in cheap unbleached cotton. America kept the cloth's name for the practice, while Britain calls the same trial garment a toile and the cloth calico.
What made historical Dhaka muslin special?: Hand-spun yarns of almost impossible fineness, woven into cloth so light it earned names like woven air. The handloom trade collapsed under industrial competition in the nineteenth century, and the modern word drifted to mean ordinary cheap cotton.

Full entry: Muslin Full entry: Calico

Sources & References

1.Muslin, Wikipedia
2.Muslin, Encyclopaedia Britannica
3.Calico, Wikipedia
4.Calico Acts, Wikipedia