The early stage of the Terran mirror match up may seem complicated to many (myself included) due to the numerous interactions of various TvT builds, and this makes it interesting and frustrating at the same time. This post discusses about the core concept of “offender versus defender” that underlies how TvT early game is played out.

Let me start with a common scenario. Often you execute your planned build the best you can, and opponent’s build interacts with yours in a weird way that one side just loses on the spot. For example, one side goes for a Hellion and Marine drop, while the other goes for a Banshee build. It seems like a typical “build order lost”, but the result is not as random as it looks. Rather it is likely due to the fact that the defeated player did not understand his or her role in the early game.

Roles in general

Whether you are conscious of it or not, players usually have pre-determined roles in a match up during a specific time frame. By role I mean you are either the “offender” or the “defender”. Let’s use bio TvZ for example, in the mid game, Terran is the offender and Zerg is the defender. Terran, as the offender, will try to take down the third or the fourth base of Zerg (depending on location). Conversely, Zerg, as the defender, will try to defend it with minimal losses and counter attack. These “roles” define how both sides play the game.

Of course, there is more depth to these roles than a simple “either or” categorisation. In TvZ early game (take extreme cheeses out of the discussion), there is no clear pre-determined role. Zerg can be the offender by doing a mass Speedling attack or other forms of Baneling or/and Roach attack. On the other hand, Terran can also be the offender by doing a Hellbat attack. However, interestingly, we rarely see both players attack each other, and one of the players usually takes on a defender role instead. Why?

This is because there is a consensus that if both go on the offensive without acknowledging the other player’s attack, one player will lose because of how the two builds interact. With this in mind, the eventual loser will have to adjust his or her role from the offender to defender. Consequently, one is usually an offender or a defender.

What factors decide who is the offender or the defender then? To put in simply, how the races interact is the most important factor in deciding who plays what role during whichever time frame. There can be circumstances that sort of swap the roles around, but they basically just extend or shorten the duration of the roles played by the players for that time frame. For instance, Zerg can make more Zergling than usual and force Terran to play relatively defensive, but Terran will still move out at some point after the delay.

Roles in TvT

Unlike the non-mirror match ups, TvT does not have pre-determined roles based on the respective races. Thus, the key factor that affects the roles of the players is the openings chosen. If you play a Command Centre first opening, you are almost certain to be the defender. In comparison, a gas first Banshee opening should put you in the offender role. I will focus on the early to early mid game time frame in this post, as it is hard to nail down who should be in whichever role in mid game and beyond since there are too many variables to account for.

Openings

If you have been following my blog, you should know that I conceptually differentiate “build order” and “opening” for implication reasons. As mentioned in the last paragraph, I used the term “openings” instead of “build orders”, because openings have a stronger direct impact on the roles adopted. Here is a list of TvT openings in general,

• Gas first (single gas)
• Gas first (double gas)
• 12 Barracks 12 Gas
• 12 Barracks 13 Gas
• 12 Barracks 15 Gas
• Macro proxy (e.g., proxy Marauder)
• Command Centre first

As usual, this list is not exhaustive, but it is probably close. Some of these openings are relatively clear in terms of what role you play, for example, Command Centre first is the defender and macro proxy is the attacker. Others, for instance, 12/12 Reaper expand is relatively unclear, as your role is not pre-determined but is dependent on opponent’s opening. If opponent is going for a gas first (single gas) opening, opponent is likely to be going for either a three Hellions, Marines and Medivac or a two Hellions, one Widow Mine, Marines and Medivac attack. In that case, the Reaper opening has to be the defender no matter which exact build opponent is going for. This is because when the attack commences, the Reaper player’s units cannot match the gas first (single gas) player’s. Let say the Reaper player swap the Factory onto the Reactor after four Marines (i.e., the most common variation), he or she will have four to five Marines, two Hellions and a Reaper at that point of time. On the other hand, opponent will have six Marines, two Hellions, a Medivac and a Widow Mine or a third Hellion. Therefore, the Reaper player has to use the defender advantage to battle against the gas first player. By the time the gas first player (offender) moves across the map, the Reaper player (defender) will have the second pair of Hellions ready and a Viking or Medivac close to complete (usually Viking). This is the classic defender rally advantage. Further, the defender can initiate the engagement when the offender is offloading the Marines, and this is another defender advantage in TvT early game.

Alternatively, let’s imagine that the Reaper player is on an auto-pilot mode, and does his or her own Marine, Hellion and Medivac attack. The gas first player will just destroy the Reaper player’s base, and it will become “just one of those games”. However, it is an accountable mistake on the Reaper player part to not know the role required of him or her due to opponent’s opening. One’s understanding of his or her role based on how his or her opening interacts with the opponent’s separates the great players from the good ones.

Perhaps you are now thinking, “well, there are too many plausible opening interactions, and it is close to impossible to know everything.” While the concept of offender versus defender may seem overwhelming for TvT early game, there are rules of thumb to follow.

Whose initiative

The first of rule of thumb is to acknowledge who has the initiative to decide the roles.

There is almost always one player that has the initiative to decide whether he or she wants to be the offender or not (unless you do mirror builds). In the above example, the gas first player has the initiative, and decides to be the offender. The other player has no choice, but to take on the defender role because of the reasons discussed above. But then, how do you know who has the initiative? The answer is, and this is extremely important, the player who puts down the Starport first usually has the initiative. This is so important that I should quote it.

The player who puts down the Starport first has the initiative.

This is based on the fundamental understanding of TvT that you have to go for a 1-1-1 build, no matter which opening you take. I won’t go in depth about this basic, as it is outside the scope of this post. Anyway, it is a consensus that you need to have a Starport in your opening for TvT, simply because of the nature of the match up’s early game.

For example, a gas first Banshee opening versus a 12 Barracks 13 Gas Banshee opening. The gas first opening puts down the Starport earlier, and hence, the Banshee completes and attacks earlier. If both players send their Banshees to opponent’s base, the Banshee the does more damage will win the game. In this case, the player whose Banshee starts to deal damage earlier wins. This is a key reason why we rarely see 13 gas Banshee opening.

Therefore, one of the most important things to scout for in TvT is the first Refinery timing, as this indirectly tells you whether opponent is able to put down the Starport earlier than you do, and that in turn tells you who has the initiative. With that being said, you may need to deduce further to know whether opponent’s opening will put down the earliest Starport possible even though you scouted gas first. The gas first Reaper opening (11 gas or 12 gas) delays the Starport slightly, and usually takes on the defender role.

First Starport unit

It is important to note that having the initiative does not mean that you have to be the offender, it just means that you have the initiative to choose. This leads to the second rule of thumb, what is the first air unit produced from the Starport?

The first air unit produced by the player with initiative decides who is the offender and who is the defender. Again, this is so important that I should quote it.

The first air unit produced by the player with initiative decides who is the offender and who is the defender.

In normal circumstances, a Banshee or Medivac signals that the initiator is the offender, while a Viking or Raven signals that the initiator is the defender. If the opponent, who is the initiator, goes for a Banshee or Medivac, you have no choice but to go for a Viking or Raven. I am quite certain that many of you have built a Medivac, and then a Banshee flies in. The game basically ends there, because you did not acknowledge who has the initiative. That had happened to me before.

With that being said, what I just mentioned is focusing on the “immediate” early game. That is, the initiator can get a defensive unit, say Raven, and builds up for a strong attack in the early mid game. Some of these common attacks are mainly Tank and Viking push or Auto-Turret Tank drop. Nevertheless, before that attack timing happens, that player has to be defensive, and this indirectly passes the offender role to the opponent for the time being.

Mirror openings

Things get complicated when both players are doing the same opening. The subsequent decisions may be down to the spawn locations, mind games, game theory and luck. Let say both players go for gas first (double gas), the first Starport unit is usually either a Banshee or Raven. Let’s use game theory,

Scenario 1: Banshee vs. Banshee (Even)
Scenario 2: Banshee vs. Raven (Raven has the advantage)
Scenario 3: Raven vs. Raven (Even)

The best choice is therefore a Raven. This also suggests that the player who doesn’t scout is rolling the dice. However, sometimes the game theory outcomes are much more complicated, that you really have to “roll a bit of the dice”. Let say both players go for a Reaper expand or a 15 gas expand. Since no one has the clear initiative, it is hard to decide what is best decision. These openings allow too many options that require different reactions from the opponent. For example, you will need to have Missile Turret or Raven to defend Banshee, but such investment may result in not being able to hold Marines, Hellions and Medivac variations well. Further, if both go for Hellion and Marine (relatively common), the player who attacks is at the disadvantage most of the time due to defender advantage. This is the very reason why I always tell my friends that Reaper opening is arguably the most difficult build to do in TvT.

Summary

It is extremely important to understand the concept of offender versus defender (not sure if I’m the pioneer who come up with this terminology). The assigned role is down to the openings chosen by the players, and usually there is one player who has the initiative to decide who takes on what role. The initiator is usually the one who puts down the Starport first, and the first unit from the Starport signals the role selected by the initiator.

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