We’ve said it for sure, and maybe you have too. A track sounds wide. We mean expansive and takes up space. We mean it as a compliment, but the thing is, even though we know what we mean and other people know what we mean, it doesn’t really mean anything. Width isn’t something there is a dial for; width is something that happened with the most calculated minds and hands. Somebody on the mix knows exactly what needs to happen around the 40 milliseconds line.

The guitar on your favorite track that feels so room-filling isn’t as enormous as you think it is.

You are in a room. Someone speaks. The sound reaches you directly, and then it reaches you again a fraction of a second later off the wall, the ceiling, the floor, the table.

By any sensible reading, you should hear four or five people - the sound is bouncing around and delayed. But you hear one.

Your auditory system takes everything arriving inside a short window after the first sound and folds it into a single event, then localizes that event to wherever the first arrival came from. Everything after gets absorbed. Not ignored, absorbed. It still contributes loudness, size, and a sense of space. It just does not get to be its own sound.

This is the precedence effect. Engineers usually call it the Haas effect, and the two are not quite the same thing. Haas is the special case: between 10 and 30 milliseconds, the copy can be up to 10 decibels louder than the original, and your ear still can’t be bothered with it.

Get it right, and a single instrument occupies the whole room. Get it wrong, and you have made an echo. Get it disgusting, and the listener cannot tell you what changed, only that the song got bigger, wider, like it could wash over you a thousand times.

That is the useful part. Not the number. The fact that the effect is invisible to the person it is working on.

It is easy to think the width comes from sound being doubled or more. Because surely if you hear it once, then twice, that means there is extra? Well, doubled means someone played it twice, on purpose. Widened means one performance, copied, with the copy pushed back a few milliseconds.

They actually don’t even sound the same. And as always, we come with some fun things to try, so you can find out: is it wide or is it doubled?

Pitch disagreements: You’re going to be tuning in, eyes closed and listening very intensely for this one. Two takes are never going to be exact matches, so if the track has just been doubled up, you’ll process this as thick, not wide. Haas will make something huge without making it heavy.

End not the start: A perfect copy releases in parallel with itself - it is the same event. The end notes of something doubled up will split and drift apart.

Sum to mono: A super fast way to know, and probably something you’ve experienced already. Find the mono slider in your settings, or just take an earbud out. A doubled track will stay solid and get louder. A Haas-widened part will give you that thin, weak, almost underwater sound. Why? Well, the copy is fighting with the original now - but you’re only hearing one side of it.

Thickness is a second person, whereas width comes from a copy.

Our ears are committed to hearing something once, but the time difference between that happening and not is very, very small. Imagine a line at 40 milliseconds. Below it, the delay disappears, and you get size. That merging is called fusion. Above it, the delay shows up, and you get space. And it is your job to decide which one you want more, and how to make it happen.

ADT, Abbey Road, 1966. Ken Townsend built it so Lennon could stop singing everything twice. Signal off the tape, through a second machine, back late, wobbling so it never settles. One take, two voices, all over Revolver.

The stereo guitar that is one guitar. Duplicate, delay one side fifteen milliseconds, pan hard apart. Spans the whole field, and you still hear one guitar in one place. Free, instant, and why records collapse and sound like trash when played out of a laptop.

Actual double tracking. Two takes fighting to be heard for three minutes. Grunge feeds off it. My Bloody Valentine stacked takes until that argument became the timbre. Nothing on Loveless is wide. It is thick, which costs more.

Slapback. Sun Studio, early fifties, tape delay at a tenth of a second. Past fusion and meant to be heard. Sits as a second event, just behind the singer's head.

Dub. King Tubby, 1972, Byron Lee's cast-off MCI desk on Dromilly Avenue, single parts through a Fisher spring reverb and tape echo. Repeats at 400 to 650ms, ten to sixteen times past fusion. Nothing merges. The move is the fader: throw a snare into the echo, pull the source. The original goes, the copies carry on.