בִּתְחִלָּה הָיָה רַבֵּנוּ הַגָּדוֹל אוֹמֵר דְּרוּשִׁים קְצָרִים בִּמְאֹד

At first, the Alter Rebbe would deliver very short teachings

מַרְעִישֵׁי הַלֵּב וּמַלְהִיבִים מְאֹד

that would arouse the heart and inflame it with spiritual emotion.

"וְנִקְרְאוּ בְּשֵׁם "דְּרָכִים

These were called derachim (“paths”). 

אַחַר כַּךְ נִקְרְאוּ "אִגְּרוֹת" וְהֵם אֲרוּכִּים יוֹתֵר

Later, his teachings became longer and were called igros (“letters”). 

"אַחַר כַּךְ הִשְׁתַּלְשֵׁל שֶׁנִּקְרְאוּ "תּוֹרוֹת

The teachings were further developed and were called Toros. 

וְהֵם שָׁרְשֵׁי הַדְּרוּשִׁים שֶׁבְּתּוֹרָה אוֹר וְלִקּוּטֵי תּוֹרָה

These became the root and core material for the maamarim in Torah Or and Likkutei Torah. 

"אַחַר כַּךְ אֲרוּכִּים מְעַט יוֹתֵר וְנִקְרְאוּ "כְּתָבִים

Later they became slightly longer, and were called kesavim (“writings”).

וְהֵם בְּבֵאוּר בַּהֲשָׂגָה רְחָבָה לְפִי עֶרֶךְ

These included explanations that allowed for relatively thorough comprehension.

Text and Translation courtesy of Sichos in English

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