Diameter denotes that we are talking about a circle as where square inches would mean we are talking about a different figure.
Units of length
Many different units of length have been used around the world. The main units in modern use are U.S. customary units in the United States and the Metric system elsewhere. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.
The base unit in the International System of Units (SI) is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second." It is approximately equal to 1.0936 yards. Other units are derived from the metre by adding prefixes from the table below:
A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil (one thousandth of an inch). It corresponds to 5.067×10-4 mm². It is a convenient]citation needed[ unit for referring to the area of a wire with a circular cross section, because the area in circular mils can be calculated without reference to pi (π).
The area in circular mils, A, of a circle with a diameter of d mils, is given by the formula:
The talking drum is an hourglass-shaped drum from West Africa, whose pitch can be regulated to mimic the tone and prosody of human speech. It has two drumheads connected by leather tension cords, which allow the player to modulate the pitch of the drum by squeezing the cords between his arm and body. A skilled player is able to play whole phrases. Similar hourglass-shaped drums are found in Asia, but they are not used to mimic speech, although the idakka is used to mimic vocal music.
Hourglass shaped talking drums are some of the oldest instruments used by West African griots and their history can be traced back to the Yoruba people, the Ghana Empire]citation needed[ and the Hausa people. The Yoruba people of south western Nigeria and Benin and the Dagomba of northern Ghana) have developed a highly sophisticated genre of griot music centering on the talking drum]citation needed[. Many variants of the talking drum exist, with essentially the same construction mentioned above. Interestingly, this construction is limited to within the contemporary borders of West Africa, with exceptions to this rule being northern Cameroon and western Chad; areas which have shared populations belonging to groups predominant in their bordering West African countries, such as the Kanuri, Djerma, Fulani and Hausa.