Nearly all fittings in the leather trade are described by the size of the straps which fit into them. The main exceptions are split rings which are used for key rings and are described by their outside diameter.
A Buckle is described by the size of strap which will fit into it. i.e. a 1½” wide strap will need a 1½” buckle.
A “D” Ring is described by the internal length of the flat side of the D.
An “O” Ring is described by the internal diameter
Eyelets are described by traditional numbers. No “200” is 4mm internal diameter. No “500” is 5.5mm internal diameter. No “700” is 6.5mm internal diameter.
Sail Eyelets are described by a different traditional system. No“18” is 5.16mm internal diameter. No”20” is 6.75mm internal. No”22 is 7.94mm internal. No”24” is 9.53mm internal. No”26” is 11.11mm internal. No”28” is 12.7mm internal.
Trigger Hooks and all other dog lead hooks are described by the size of strap that will fit into the loop. The only exception is the double ended trigger hook which is described by it’s overall length.
Sizes of Threads
Linen Threads are described with two numbers. i.e. 18/5 or 25/3 etc. The first number is a guide to the thickness of each strand and the second is the number of strands in the twist. N.B. the smaller the first number the thicker the strand. ( the system is derived from a standard measuring ring which would contain a lot of thin strands or fewer thick strands )
Bonded Nylon Threads are described with a metric system. Once again the smaller the number the thicker the thread.
Wax Twins are described with two numbers i.e. 72/8. The first number is the overall length in inches and the second is the number of strands in the twist. Because the strands are all the same thickness there is a direct relationship between the second number and the thickness. eg. 72/8 would be twice as thick as 72/4.