user16626: After 1.e4 c6, 2. d4 d5, the move 3. Nc3 is called the "mainline". According to lichess's database, the move 3. e5 has been played more than 3. Nc3, both by masters and by non-masters. This makes me wonder why we don't call 3. e5 the "mainline".
In general, what makes an opening move the "mainline"? Is it not the most popular move?

user18809: There's no universal definition of what constitutes a mainline move. It also changes through time, so in this case 3.Nc3 may have been the mainline before but not anymore.
Anyway I don't think I've heard "mainline Caro-Kann" to refer to 3.Nc3, but rather "Classical Caro-Kann".

16626: <p>After 1.e4 c6, 2. d4 d5, the move 3. Nc3 is called the &quot;mainline&quot;. According to lichess's database, the move 3. e5 has been played more than 3. Nc3, both by masters and by non-masters. This makes me wonder why we don't call 3. e5 the &quot;mainline&quot;.</p>
<p>In general, what makes an opening move the &quot;mainline&quot;? Is it not the most popular move?</p>
 18809: <p>There's no universal definition of what constitutes a mainline move. It also changes through time, so in this case 3.Nc3 may have been the mainline before but not anymore.</p>
<p>Anyway I don't think I've heard &quot;mainline Caro-Kann&quot; to refer to 3.Nc3, but rather &quot;Classical Caro-Kann&quot;.</p>
