Thread Pitch and Threads Per Inch are both used to measure the threading of a bolt or nut to ensure that they are going to couple together properly. ¹⁾
If the threading of a bolt and nut are different, they will either seize or strip the threading resulting in an unusable connection.

Threads per inch, commonly abbreviated as TPI, is a term frequently used in US fasteners and is a term used to help identify how many threads are in an inch.
To determine treads per inch, an inch of the threads are measured longways and then the peaks on the fastener are counted within that inch.
For threads shorter than 1 in, you can measure half an inch of threads and double the result to get the TPI.

Thread pitch is a term used in place of TPI in metric fasteners and is the distance from one thread to the adjacent thread measured longways across the thread peaks.

US threads generally have 3 numbers although the last can be left off unless required.
The first number is a whole number and/or fraction, the second number is the threads per in (TPI), followed by the length in inches.
The diameter is the outside diameter of the thread peaks.
In this example of a 1/4“ bolt bolt; 1/4” x 20 x 1-1/2“ states that the app. diameter of the threads is 1/4”, the TPI is 20 and it's 1-1/2“ long.

Metric bolts are written down as an “M” followed by 3 numbers and again the last can be left off unless required.
The first number is the diameter, the second the thread pitch, and the third the length in millimeters. ²⁾
The diameter is the outside diameter of the thread peaks.

In this example of a 10mm bolt; M10x1.25×35 states that the diameter of the threads is 10mm, the thread pitch is 1.25mm and it's 35mm long.

The most common sizes for brake line connections are 3/8”-24, 7/16“-24, 1/2”-20, 5/8“-18, 10mm-1.0 and 12mm-1.0. ³⁾

3/8” x 24 vs M10 x 1.0mm
3/8“ x 24 TPI equates to a thread pitch of 0.0416666666666667” or 1.05833333mm.
3/8“ x 24 stated in metric units equates to 9.525mm x 1.05833333mm
So a 3/8” fitting is slightly smaller in diameter than the 10mm fitting but each thread is .058mm farther apart than a 10mm fitting's threads.

A 3/8“ x 24 UNF American nut will thread very easily and loosely into a 10mm x 1.0 thread fitting but will most likely strip the threads when you tighten it down. ⁴⁾
You can start a 10mm x 1.0 threaded nut into a 3/8” x 24 UNF thread fitting but it will immediately begin stripping the threads.
It will catch the first thread, making you think that it will go in, but it will destroy the threads as you tighten it.

NPT stands for National Pipe Thread. Descriptions are commonly used as either straight (NPS) or tapered (NPT). ⁵⁾
Pipe thread sizes do not refer to any physical dimension.
NPT has flattened peaks and troughs where BSP is rounded. NPT has a thread angle of 60º where BSP is 55º.

There is no direct naming relationship between the measured diameter of NPT pipe threads and the corresponding NPT size.
The measurements are “trade sizes.” That means they are uniform sizes according to the diameter of the pipe or fitting that they can be used with.
The outside diameter of each pipe fitting can be measured and compared to the table below for size identification. ⁶⁾
The OD measurement for a male connection should be taken near the base of the threads, farthest from the (tapered) end of the part.
An ID measurement for female connection should be taken as close to the opening as possible while remaining within the threads.
An OD measurement for a female connection is more difficult, but sometimes may be taken by measuring the outermost edge of the threads at the pipe opening.

• The sizes in the chart below are the major dia of the male straight thread (as the pipe OD is the major).
  And the major dia. of a tapered male thread at it's big end. ⁷⁾
• The tapered thread has it's diameter reduced on a taper of .0625 per inch of thread length. Almost all hardware store fittings will be tapered thread.⁸⁾
• Little known fact: All iron heads pipe threads used in cases, rocker boxes, and oil pumps are straight female thread, fitted with tapered thread male fittings.⁹⁾

AN (or Army/Navy) was never intended to characterize an entire fluid system, and is mostly designed as a thread specification.
AN sizes range from -2 to -32 in irregular steps, with each step equating to the OD (outside diameter) of the tubing in 1/16“ increments.
This system does not specify the inside diameter of the tubing because the tube wall can vary in thickness. ¹⁰⁾

• You can measure AN threads by either using a caliper or simply laying a fitting across a tape measure.
  Measure the OD of the threads and count the number of threads per inch to confirm with the chart below.
  Thread OD will not exactly match the SAE thread size but will get you close enough if you have a AN fitting.