If you've spent any time at all playing Machinist, Summoner or most Melee jobs - you've probably at least heard the term "Double Weave". Despite the prevalence of the term, the game itself is completely lacking in explanation and likely because of this, general understanding of the execution, concept or both is lacking.
With this guide, I attempt to demystify the concept of double weaving to better help you master your job!
Section 1: Terminology
Before we dive into the hows and whys, it's important to understand some key terms we'll be using - namely GCD and oGCD.
GCD stands for Global Cooldown - it refers to the spells, weaponskills and abilities you have that all share a recast timer. When you use a GCD you'll notice that all other GCDs will go on cooldown at the same time, as illustrated below:
A GCD can be instant, going off as soon as it is pressed - like Machinist's Split Shot, or it can have a cast time - like Summoner's Ruin III. The only thing that makes it a GCD is whether it affects the use of other actions on the global cooldown.
oGCD stands for off-Global Cooldown - as the name implies, it refers to actions that do not share a Global Cooldown. These abilities can generally be used at any time without affecting other abilities, and are almost always "instant". I put instant in quotation marks because in reality, there's about three quarters of a second before you can press another ability (often a little more if you live far away from the FFXIV servers). See below for a visual reference:
oGCDs can have long or short cooldowns and do not share a cooldown with any other ability unless specifically noted in the tooltip.
Clipping refers to when you can't use a GCD that just came off cooldown because you've just used an oGCD. See below:
Notice how after using the oGCD Egi-Assault and trying to cast the GCD Ruin II immediately after, the action doesn't go off for quite a while despite several presses. That is because the oGCD clipped into the GCD time window.
Section 2: Weaving and the GCD Cycle
In order to understand what it means to double weave, understanding what weaving means in the first place is a must.
You may have noticed that if you use an instant GCD, you have time to use an oGCD before the global cooldown has finished. Effectively fitting the use of oGCD skills in between your GCDs is called weaving.
"But why is this important?" you might be thinking. "I can just use oGCDs at any time, so why does it matter if they're between GCDs or not?"
The answer to that is three parts:
A) Your GCD abilities are generally the largest part of your damage. Even if their overall potency is lower than other abilities, the fact that you can keep using them over and over adds up.
B) For most jobs, using your GCD abilities confers special buffs or bonuses that you need to be effective in combat (such as Dancer's Fourfold Feathers) - or takes advantage of them (such as Summoner's Ruination).
C) It's the most effective use of time. Instead of just spamming all of your oGCDs at once, using your GCDs as often as possible still lets you fit in all of your oGCDs in between them. You're still getting off all of those spells, but with tons of extra GCDs as well - meaning a lot more damage.
As you might infer from this information, that means that using your GCD as often as possible is the key to doing the most damage - stopping it to use more oGCDs is a negative to your dps - and there we have the need for weaving.
Section 3: Double Weaving
At this point, you may have guessed if you didn't already know, but Double Weaving refers to fitting two oGCD skills between your GCDs.
"Why two? Why not three, or four?" Good question - remember that "instant" in quotation marks from earlier? Because of that 0.7 second or more delay when using an oGCD, we can only fit two oGCDs after an instant-cast GCD without clipping into the next one (See Section 1 if you don't remember what that is).
Properly fitting two oGCDs after a GCD cast is the entire concept of Double Weaving. It sounds simple, but takes practice. When you're doing it right, you're pressing 3 buttons in under 2.5 seconds - it's a lot to do!
You should note that there are some edge cases where you just can't double weave without clipping. These include using Infusions/Potions and some abilities with long animations like Dragoon jumps. I recommend consulting guides on The Balance to properly understand which actions to double weave and which not to.
I hope this guide has helped you better understand what it means to Double Weave, and how to effectively execute it. I'll leave you with a demonstration of a Summoner opener full of double weaving as a final example:
If you watch the hotbars, you can see how there are two oGCDs weaved between each GCD cast - with the exception of the use of an Infusion, which has a long animation and must be single weaved.