If as White you open with 1. e4 you must have a ” Main Weapon”

with which you will meet 1. … e5

The most common choice is either the Two Knights or Ruy Lopez

You must also have a reply to the Petrov’s Defence  1.e4 e5 2.Nf3 Nf6 and the Latvian Gambit 2. … f5 or Elephant 2. … d5

If as White you open with 1.e4 you must be prepared to face –

 

[a] 1. … Nf6   Alekhines Defence

[b] 1. … c6    Caro-Kann Defence

[c] 1. …  e6    French Defence

[d] 1. …  c5    Sicilian Defence

[e] 1. …  d6 2.d4 Nf6  Pirc Defence

[f]  1.  … g6 2.d4 Bg7  Robatsch Defence

[g]  1. … d5 2. exd5  Center Counter [ or Scandinavian] Defence

[h]  1. … Nc6  Nimzovitch Defence

[i ]  1. …  Various other less common moves

of these the most frequently played are the Sicilian and the French

 

As Black against – 1.e4  1.d4  1.c4 and most other first moves

you can reply with 1. … d6 and … g6 with … Bg7 using

Variations of the Pirc or Kings Indian Defence