Jay Barnson on Turn-Based Combat and its Variants

The latest blog from Jay "Rampant Coyote" Barnson attempts to catalog the various types of turn-based mechanics in RPGs and strategy games. While there's certainly room for debate on these definitions and categories, it's a good starting point, especially for people who are unfamiliar with the intricacies of turn-based mechanics:
Turn-Based With Reactions:
This is another variant that behaves very much like a classical turn-based system, but the (non-active) players are able to react to the active player's moves under certain condition. This could be as simple as automatic (reaction fire) taking place when a unit breaks cover in front of an enemy unit, or a more complicated reaction requiring the inactive player to make a decision (if only to choose (do I take this attack of opportunity or not)). The major advantage of this kind of system is more realistic rules to limit the gaming of turn-based limitations, but it can also help keep inactive players engaged when it is not their turn.

Phase-Based:
This variant breaks a turn into distinct phases that resolve independently. Players alternate actions at the phase level, rather than at the turn level. For example, a game might have a movement phase separate from a fighting phase, so that all players move first, and only then do they resolve attacks. This approach can reduce the advantage of going first (or last, in some game systems), and it shrinks the individual stretches during which a player is inactive. In at least one game I've played (Supremacy, a board-game from the 1980s), players could only choose to participate in some of the phases, skipping the rest. This usually meant that a player actively engaged in warfare with another country couldn't fully participate in the economic development phases. Wizardry and The Bard's Tale (original) games are often described as phase-based games, with a command phase followed by a resolution phase, though these could also be considered a variation of Simultaneous Resolution (see below).

Activation-Based:
In this variant players take turns moving (activating) units (usually one at a time or two at a time), until all units have had their turn. Players can choose which units go at which time, which can present all kinds of interesting tactical opportunities. It provides somewhat more realism than classic turn-based systems and keeps the players engaged with very short rounds. However, said (tactical opportunities) can also lead to (gaming the system.)

As a point of personal interest, Frayed Knights 1: The Skull of S'makh-Daon uses a Unit Initiative based system, while Frayed Knights 2: The Khan of Wrath is using something far closer to an Activation based system.

Unit Initiative-Based:
This variation of turn-based also has players moving units individually and alternating actions during the course of a turn, the order is determined by an initiative system for the individual units, rather than letting the player decide over the course of a complete turn. In other words, during the course of a turn, all units on all sides have an order in which they become active and can be moved. This is the classic approach of Dungeons & Dragons and older, party-based games either directly licensing or inspired by the system.

Head over to Jay's blog to read the full list!