Most leather trade fittings are described
by the size of the straps which fit **into them**.

The main exception to this is the split
rings used for key rings which are described by their **outside diameter**.

**So:**

A buckle is described by the size of strap which will fit into it. i.e. a 1.5" wide strap will need a 1.5" 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.

However for
double ended trigger hooks they are described by 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.**Note:** the smaller the first
number the thicker the strand of thread. This system is based on a standard measuring
ring which would contain a lot of thin strands but 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 - e.g. 72/8 would be twice as thick
as 72/4.