They helped one another with the preparations.
Here, the underlined part is an example of -

• They helped one another with the preparations.
- Here, the underlined part is an example of- Reciprocal Pronoun.

• Reciprocal Pronoun:
- যে সকল Pronoun সাধারণত পারস্পারিক সম্পর্ক নির্দেশ করে তাকে Reciprocal Pronoun বলে।
- Reciprocal Pronoun গুলো হচ্ছে: Each other, One another.
- Each other: দুইজনের মধ্যে বুঝাতে each other ব্যবহার করা হয়।
- One another: দুইজনের অধিক হলে One another ব্যবহার করা হয়।

• Pronoun: Pronoun হলো noun এর পরিবর্তে ব্যবহৃত word, এ কারণে Pronoun কে Substituting word বলা হয়।
- Example: He, She, We, They, I, etc
.
• Pronoun ৮ প্রকার। যথা:
01. Personal Pronoun: I, me, my, mine, we, us, our, ours, you, thee, your, yours, he, him, his, she, her, hers. it, its, they, them, their, theirs.
02. Reflexive Pronoun: myself, ourselves, yourself, yourselves, himself, herself, themselves, etc.
03. Demonstrative Pronoun: this, these, that, those, so, such, the same, etc.
04. Indefinite Pronoun: one, any, some, many, all, few, several, no one, everyone, none, someone, nobody, anybody, somebody, another, nothing, etc.
05. Interrogative Pronoun: what, which, who, whom, whose, etc.
06. Relative Pronoun: what, which, who, whom, whose, that, as, but, whoever, whoso, whomever, whatever, etc.
07. Distributive Pronoun: each, either, neither, etc.
08. Reciprocal Pronoun: Each another, One another.

Source: A Passage To The English Language S. M. Zakir Hussain.