The purpose of this study was to identify and introduce some Iranian dialectal words and word structures to enter Persian language to develop its lexical system. Persian is the formal language in Iran; spoken all over the country; used in media, schools, universities, trade, etc. Tati is one of the wide spreading Iranian dialects spoken from the northwest in Azerbaijan to the northeast in Khorasan. It is also used by the people in some central provinces. Among many variants of Tati in Iran, the researchers selected one that is spoken in the small town of Jirandeh Amarlu, Guilan Province. Some Tati words containing nouns and adjectives were chosen from the glossary book of Giltat (2018), and compared with Persian word structures which Bateni (1970) had analyzed based on the Scale and Category Theory. This comparison was shown in some word components tables along with the essential explanation for each table. The result of the study showed that many word structures in both languages are the same. These structures make common and dialectal words to meet the needs of the speakers in Persian and Tati. Some of these dialectal words have the ability to enter Persian, because of their easy pronunciation or conveying a new meaning. Some structures were found in Tati which do not exist in Persian. These structures can also enter Persian to make words in this language. They can increase the number of words of the formal language and make its lexical system larger.

1. Introduction
Persian and Tati, like many other cognate Iranian languages, are agglutinating. It means that most words of these languages are made by joining lexical or grammatical morphemes to each other. The lexical system of all dialects of this group has some word structures in common with formal Persian language. They make common words with Persian some of which have dialectal usage and meaning. These dialectal meanings can also be used by Persian speakers in order to increase the semantic spread of existing words in this language. These dialects have also their own word structures, too. They make dialectal words to meet their own speakers’ needs. Some of these dialectal structures may be able to produce words in Persian, either. Even some dialectal words which have semantic load and fit to the phonology system of Persian can also enter this language. Comparative study of word structure in various categories including: noun, verb, adjective, etc. is a simple and practical way to discover these dialectal abilities to strengthen the lexical system of Persian language.
The aim of this study is to search in Tati dialect and find some word structures which do not exist in Persian, but they have ability to enter Persian and make words in this formal language, too. Some dialectal words of common structures which are easy to pronounce for Persian speakers can be suggested to enter, either. Some other terms which have a good semantic load but they less/do not adapt the phonological system of Persian, can also be entered if they are Persianized or translated. In these ways the lexical system of formal Persian language will be improved by using the words of its cognate dialects.
Research questions
The main research questions of this study are:
a.    How can we take advantage of the capacity of Tati word formation processes to strengthen Persian language?
b.    How can we gain more knowledge about word formation processes in Persian language by comparing Tati and Persian lexical systems?
1.    Literary Review
The theoretical framework adopted for this study is based on Scale and Category Theory. This is one of the views of Halliday, a prominent contemporary linguist, and has proposed materials in the word structure section that can describe the compound structure of Persian language and other Iranian languages much better than other theories. Based on this theory, Bateni (1970, pp. 178-179) introduces the general formula [prefix(s) + base(s) + suffix(s)] for the structural components of the word in Persian, and analyzes a number of words in some word components tables. In this formula, the prefix(s) and suffix(s) are optional, but the base is mandatory. The plural form of the components is a sign of the possibility of repetition. The prefix and the suffix classes, which are placed in the ‘p’ and ‘s’ positions in the tables, are composed of bound morphemes; but the morpheme class that is in the base position includes both bound and free morphemes. Bases that are free are marked with two lines and those that are bound are marked with a line below. 

2.    Methodology
This study adopted the comparative-descriptive method because the aim of the study was to find some dialectal Tati structures and vocabularies to enter Persian language to make its lexical system larger. To reach this aim some Persian nouns and adjectives analyzed in word components tables based on Scale and Category Theory by Bateni (1970) have been chosen; their word structures have been explained; then, a comparative study has been done between these Persian words and Tati words existing in the glossary book of Giltat, volume 2 (2018). We have used the same method in parsing Tati words into its components in some tables to show their structural differences and similarities. Through this study, we have introduced the dialectal words that can directly or indirectly adapt with the phonetic system of Persian language to enter this system. The variance of Tati whose words have been studied here is spoken in the small mountainous town of Jirandeh Amarlu, on the west side of Alborz, in Guilan province. 

3.    Results
The results of this study show that the lexical system of Persian language can be improved by using Iranian dialects through the following ways:
1.     There are some dialectal word structures in Tati which are not found in Persian. These structures can enter Persian and make words in this language, too. Some dialectal words of these structures can also be used in Persian and increase its word number.
2.     There are some common word structures which make words in both languages. Some of these dialectal words can also be used in Persian conditionally.
Directly: words that have enough adaptability with the phonological or grammatical system of Persian can enter Persian with no change.
Indirectly: Some dialect words do not have enough adaptability with the phonological or grammatical system of Persian, but they are valuable words because of the meanings that they convey.  These words can enter Persian with a slight syntactic or phonological change or even through translation