The instrument has two hinged limbs with steel points, each limb being built up of three riveted brass plates. The middle plates of the sandwich are approximately half the width of the others, making the limbs' inside edge hollow. Fixed to one of the limbs is a brass circular arc. The instrument closely matches the illustration and description published in Thomas Hood's The Making and Use of the Geometricall Instrument, called a Sector (London, 1598), suggesting a date of circa 1600.
On one face, the limbs carry sectoral scales of equal parts from 0 to about 113, divided to 10 and subdivided to 1 and 1/2 and numbered by 10. Allowing for some wear, the distance from the sector's centre to the tips of the points is approximately 120 units, as recommended by Hood. The other face carries two sets of scales, one for the chords of circles divided and numbered from 3 to 10 and the other for squares (or powers, as they are referred to by Hood) divided and numbered from 1/2 to 1/8.
The circular arc (or 'circumferentiall limb') carries on one side a scale of degrees up to 150, divided to 10, subdivided to 5 and 1, and numbered by 10. This scale is further subdivided by diagonals to 10 minutes. A scale of polygons divided and numbered from 3 to 10 is also included, though this is not mentioned by Hood. On the reverse are three scales used to measure distances between the points of the sector. The scales can measure up to 17 inches and are graduated to read in 6, 8 and 10 parts of an inch. The first (inner) scale runs to 102, divided by 6, subdivided by 2, 1 and 1/2, and numbered by 6; the second runs to 136, divided by 8, subdivided by 4, 2, 1 and 1/2, and numbered by 8; the third (outer) scale runs to 170, divided to 10, subdivided to 5, 1 and 1/2, and numbered by 10.
The reverse of the arc also carries two further features not listed by Hood. Inside the inch scales is an additional scale graduated from 1 to 32, divided to 1, subdivided to 1/2 and 1/4. The scale gives approximately equal parts between the points but does not have the same origin as the inch scales. Beyond the inch scales there is also an artillery table drawn up to record the size and weight of shot and the amount of powder required for different types of artillery ('Height of bullot', 'Weight of shot' and 'Quantety of pouder'). The table's data were never entered but 'The names of peeces' were listed in order of decreasing size: 'Dubble Canon, Dub Can of Franc, Demi Canon, De Cano of Fran, Culuerin, De Culue, Sacar, Minion, Faucon, Fauconet'.
Hood's description of the sector includes 'accidentall' as well as essential parts. The additional sliding index limb does not survive on this example, nor do the sights and mounting pieces. However, each limb has a hole near its point and there is also a hole at the centre of the hinge. These would have received the recommended three sights. A square hole in the movable limb could have been used to fix the instrument on a stand.
Although Hood's book on the sector advertised the instrument's maker as Charles Whitwell, this example has been attributed to another London maker, James Kynvyn, by G. L'E. Turner on the basis of the engraving style.
The instrument is illustrated and described in J. Bennett and S. Johnston, The Geometry of War, 1500-1750 (Oxford, 1996).