I started to think about if I'm really doing it right when I'm putting my characters in units in the deployment phase... and also what happens when they leave them.

If I put down a unit of warriors 5x2 (1), and then add a mounted character into said unit I usually just put in in the side, or if I want somewhere in the middle, expanding the unit into 6x2 (2). But is that right? Should I instead deploy the warriors 6+4(3), so that the units width doesn't change when the character joins (2).

I'm starting to suspect that when I deploy the unit 5x2 it will end up 5,5,2 when the character joins (4).

1
wwwww
wwwww

2
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wwwwwc

3
wwwwww
-wwww-

4
wwwwc
wwwwc
--ww--

Furthermore, what happens when the character leaves the unit midgame? I usually just keep how the warriors stood, but is that right? Shouldn't the front keep it's width, and the back rank warriors fill up the missing slot?

So, a 6x2 formation with character (5), transforms to a 6+4 without character (6)

5
wwwwwc
wwwwwc

6
wwwwww---cc
-wwww-


Do you understand what I mean? Typing in it at work in a rush...

This.

Just consider how you add single base characters. They displace something in the front rank. Mounted characters just displace more.

However.

Placing a character with a different sized base beside a block is also (annoyingly) legal as per GW's loosy goosy FAQ on the subject.

Let's make that a group of 5x4 warriors joined by a mounted character.

WWWWW
WWWWW
WWWWW
WWWWW

Could add the character like so:

WWWWC
WWWWC
WWWWW
WWWWW
___WW

or like so:

WWWWWC
WWWWWC
WWWWW
WWWWW

Breaks the basic even rank rules, but GW explicitly allows the 'ride-along' character.

So in your example, a character can go into a 5x2 unit or a 6+4 unit and in different ways, end up with a 6x2 unit like so:

WWWWWC
WWWWWC

Ok, thanks. Good to know, this one.

And, that goes the other way round too I suppose, i.e. if my sidejoining mounted character leaves the unit, the warriors doesn't step up to fill the space where the character was, just stays where they are, making the unit (again) 5x2 - correct?

Seems weird, but I have lately played it that way so...

Depends on how the character joined them. :P

If you stuck the character on the side of a 5x2 unit, it has 5 wide ranks after the character leaves. If you add a character to a 6+4 unit, it keeps 6 wide ranks after he leaves.

A bit odd, but that's how it works out.

Though most folks aren't really following that close but decker is quite right.

Where it gets tricky is what happens when a character joints a unit five wide with one rank (cavalry for example)

Do you go 5 and 1 in the rear or six wide.

I would say six wide.. but you could go either way I suppose.

I know I've placed my BSB on Dark Steed just next to the Spearmen Unit at times... they're six wide by three at the moment so it follows the normal ranking conventions already and with the differing base sizes it's just a lot easier.