Brass screws are essential wherever rust would be detrimental, as in boats.
(b) Screws are also classified by shape; as, flat-headed, round-headed, fillister-headed, oval-countersunk-headed, and square-headed screws.

Flat-heads are most commonly used.

There are also special shapes for particular purposes.

Round-heads may be used either for decoration or where great drawing power is desirable.

In the latter case, washers are commonly inserted under the heads to prevent them from sinking into the wood.