From this definition, it follows that a rectangle has two pairs of opposite sides of equal length; that is, a rectangle is a parallelogram. A square is a special kind of rectangle where all four sides have equal length; that is, a square is both a rectangle and a rhombus. A rectangle that is not a square is colloquially known as an oblong.
Of the two opposite pairs of sides in a rectangle, the length of the longer side is called the length of the rectangle, and the length of the shorter side is called the width. The area of a rectangle is the product of its length and its width; in symbols, A = lw. For example, the area of a rectangle with a length of 5 and a width of 4 would be 20, because 5 × 4 = 20. See the picture above right.
In calculus, the Riemann integral can be thought of as a limit of sums of the areas of arbitrarily thin rectangles.
Oblong
The word oblong was once commonly used as an alternate name for a rectangle. In his translation of Euclid's "Elements", Sir Thomas Heath translates the Greek word ετερομηκες[hetero mekes - literally "different lengths"] in Book one, Definition 22 as oblong. . "Of Quadrilateral figures, a square is that which is both equilateral and right-angled; an oblong that which is right angled but not equilateral...".
References
Heath, Sir Thomas L. The Thirteen Books of Euclid's Elements. 2nd ed. 3 vols. 1926; rpt. New York: Dover Publications, Inc., 1956.
Since there are no standardized duct sizes, and due to the wide range of pressure and temperature combinations, each rectangular metal expansion joint is customer engineered to provide the most economical design that will not sacrifice the integrity of the expansion joint or the system in which it is installed.
It is important to understand, however, that due to the large sizes that rectangular expansion joints frequently are, lateral deflection of single bellows is often impossible.
The proper design for a rectangular bellows for these types of movement is a universal expansion joint, in which two bellows elements connected with a center duct section (centerspool) are used in tandem.
The rectangular weir is the most commonly used thin plate weir.
The U.S. Bureau of Reclamation has conducted many weir tests over several decades using weirs with particular dimensions - usually b's in 1 ft. increments up to about 10 ft. Therefore, any weir outside their tested dimensions is non-standard, and their equations should not be used.
To provide a single reliable, accurate method to model all rectangular weirs (suppressed, partially contracted, and fully contracted), the Kindsvater-Carter equation (Kindsvater and Carter, 1959) was developed.
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