How to Reply in a Thread in Slack (T)
Windows: T (with message focused via arrow keys)
Mac: T (with message focused)
Pressing T while a specific message has keyboard focus (reached by using arrow keys to move focus up and down through the message list rather than clicking) opens that message's thread reply panel, letting you type a threaded response without needing to precisely click the small 'reply in thread' link that only appears when hovering directly over that message.
**Why threading matters in a busy channel**: without threading, every reply to a specific message posts as a new message in the main channel's continuous flow, mixed in with whatever unrelated messages other people are simultaneously posting — in a high-traffic channel, this makes following any single conversation thread genuinely difficult, since a detailed back-and-forth about one topic gets visually interleaved with completely unrelated messages arriving from other people at the same time. Threading solves this by branching the entire reply conversation off into its own contained panel, keeping the main channel's timeline readable while still preserving the full detailed discussion for anyone who opens that specific thread.
**Getting keyboard focus on a message first**: this shortcut requires a message to actually have focus, which happens through arrow-key navigation within the message list (pressing Up or Down while not actively typing in the compose box moves a focus indicator through recent messages) rather than simply having a channel open — clicking directly into a message's hover-revealed reply link achieves the same destination without needing this keyboard step at all, which is the more intuitive route for anyone not already navigating by keyboard.
**What happens once the thread panel is open**: it opens as a side panel (or, depending on your Slack layout settings, potentially a different arrangement) showing the original message at the top and any existing replies beneath it, with a compose box specifically for adding to that thread — messages typed here post only into the thread, not into the main channel view, unless you specifically check the option to also send it to the channel.
**Also sending a thread reply to the main channel**: a checkbox near the thread's compose box lets you optionally also post your threaded reply into the main channel's visible timeline — useful for a reply that's specific to one message's context but also contains information the whole channel should see without needing to open the thread specifically to find it.
**Related shortcuts**: T specifically opens the reply panel; getting there via mouse instead just means clicking the small threaded-reply count/link that Slack displays beneath any message that already has replies, or the reply icon that appears on hover for a message with no replies yet.
**Mistake to avoid**: replying to a specific message's point directly in the main channel rather than in its thread, purely out of habit or because the thread-reply UI wasn't immediately obvious, contributes to exactly the kind of cluttered, hard-to-follow channel history threading is meant to prevent — building the habit of using T (or clicking the thread link) for any reply that's specifically responding to one particular earlier message, rather than starting a wholly new topic, keeps a busy channel considerably more navigable for everyone in it.