Tailor's chalk

Tailor's chalk is a type of chalk which is designed to make temporary markings on cloth. Using this chalk, a tailor can make markings where fabric needs to be cut or garments need to be altered, and the chalk can also be used to mark out cutting, hemming, and darting lines on garments as they are constructed. Once the markings are no longer useful, they can be easily brushed off or washed out, leaving no residue behind.
The advantage to this type of chalk is that it is fast and easy to use, and it leaves no trace behind. This can be very useful when doing something like fitting a suit, as it can be used to make markings as an alternative to forcing someone to stand still while the fabric is pinned. Tailor's chalk can also be used to make notations on fabric to ensure that it is handled and sewn properly as a garment is assembled.
There are a number of formats for tailor's chalk. Some companies make it in the form of a powder with an applicator, and it also comes in pencil form. Classically, tailor's chalk is simply sold in the form of a thin wedge, with people using the edge to apply the chalk to fabric. Many brands of tailor's chalk feel slightly waxy to the touch, although a waxy residue will not be left after the chalk is brushed off.
In addition to coming in classic white, this chalk is also available in a rainbow of colors. Multicolored chalk can be useful for making coded markings, and it also sometimes shows up better than white chalk against certain fabrics. You may also see tailor's chalk referred to as “sewing chalk,” and you might be interested in knowing that it isn't actually chalk at all: it is usually made from talc.
The same properties which make tailor's chalk useful can also be problematic. Because the chalk is so easily removed, it is possible to accidentally brush off the chalk when fabric and garments are handled, obscuring or obliterating the markings and requiring a new marking session to correct the error. As a result, tailors try to be careful when handling marked fabrics, and once fabric has been marked, it is usually sewn, altered, or cut as promptly as possible.