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Incandescent light bulbs come in a range of shapes and sizes. The names of the shapes may be slightly different in some regions. Many of these shapes have a designation consisting of one or more letters followed by one or more numbers, e.g. A55 or PAR38. The letters represent the shape of the bulb. The numbers represent the maximum diameter, either in 18 of an inch, or in millimeters, depending on the shape and the region. For example, 63 mm reflectors are designated R63, but in the US, they are known as R20 (2.5 in). However, in both regions, a PAR38 reflector is known as PAR38.

What do the letters identify in a bulb shape designation?
Answer: the shape of the bulb
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There are more than 100 freshwater fish species, varying from the giant European catfish (in the Tagus International Natural Park) to some small and endemic species that live only in small lakes (along the western portion of country, for example). Some of these rare and specific species are highly endangered because of habitat loss, pollution and drought. Up-welling along the west coast of Portugal makes the sea extremely rich in nutrients and diverse species of marine fish; the Portuguese marine waters are one of the richest in the world. Marine fish species are more common, and include thousands of species, such as the sardine (Sardina pilchardus), tuna and Atlantic mackerel. Bioluminescent species are also well represented (including species in different colour spectrum and forms), like the glowing plankton that are possible to observe in some beaches.

What type of glowing species can be found on some beaches in Portugal?
Answer: plankton
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In ring-porous species, such as ash, black locust, catalpa, chestnut, elm, hickory, mulberry, and oak, the larger vessels or pores (as cross sections of vessels are called) are localised in the part of the growth ring formed in spring, thus forming a region of more or less open and porous tissue. The rest of the ring, produced in summer, is made up of smaller vessels and a much greater proportion of wood fibers. These fibers are the elements which give strength and toughness to wood, while the vessels are a source of weakness.[citation needed]

Along with strength, what property do wood fibers lend to wood?
Answer:
toughness