Sarf - scale

In Arabic, root letters are called original letters ( حروف أصلية ).

A noun that is made up by root letters without any additional letters is known as
ism mujarrad ( اسم مجرد )

A noun that is added with additional letters to the root letters is known as
ism mazeed ( اسم مزيد )

There is a scale (pattern) in Arabic which is used to identify the root letters:

e.g.  رجل  (a man)

the first letter is ر
the second letter is ج
the third letter is ل

The scale will identify
the first letter as ف or فاء الكلمة (The "fa" position of the word)
the second letter ع or عين الكلمة (The "ain" position of the word)
the third letter ل or لام الكلمة (The "lam" position of the word)

By learning the scale, it is easier to identify the original letter from a derived word (example below) when looking up for the meaning by using root letters base dictionary like Hans Wehr or Aratools.

e.g.  From the derived word  مكتوب
     
       The letter م = Additional letter
       The letter ك = The "fa" position of the word ( فاء الكلمة )
       The letter ت = The "ain" position of the word ( عين الكلمة )
       The letter و = Additional letter
       The letter ب = The "lam" position of the word ( لام الكلمة )

Searched for  ك ت ب  in the root letters dictionary.