Gdtyjr

The original question is about placing figures inside a tabular and trying to adjust them like this:

with a code similar to this:

documentclassarticle
usepackagegraphicx
usepackagearray
begindocument
begintabular
 includegraphics[width=35mm]example-image-a & multicolumn2cincludegraphics[width=35mm]example-image-b & includegraphics[width=35mm]example-image-c \
multicolumn2includegraphics[width=35mm]example-image-a & multicolumn2cincludegraphics[width=35mm]example-image-b
endtabular
enddocument

That for some reason gives this output:

Why this happens?

Of course there are better options for placing the figures like that and the answer in the linked question is one of them. But trying to develop the code with the tabular will help as to understand some things about the additional cell that if will be used in compination with multicolumn command will reduce our columns and this can be unexpected!

1. 
   

   First step...

   
   
   

   Ok, I have 5 figures and I need 3 on top and two on bottom, I want to control their placement... A tabular seems a good solution to me...

   
   

Let's think:

Three on top: ccc
First row normal centered and I will split cells via multicolumn in second to share the space... But...

begintabularccc
include..&include..&include..\
multicolumn2c&multicolumn2c
endtabular

Oh! writing the second multicolumn I realized that they became 4 columns...
Don't even compiling...

Seems I need a fourth column:

Let's try:

2. 
   Second step... four columns:

I will split the top middle columns with multicolumn:

begintabularcccc
include...&multicolumn2cinclude...&include...\ %Nice... Let's finish with the next row
multicolumn2cinclude...&multicolumn2cinclude
endtabular

4 in the up row 4 in the next... perfect! Let's compile

Seems I didn't count correct... Lets count...

• Above row 4 cells middle 2 merged


• Below row 2 & 2 cells...

Why first cell of second row doesn't merge with the second? This should give the fourth figure centered in two first columns like fifth... Why?

Do I need to add a fifth column?

Lets see:

begintabularcccc
1&2&3&4\
include...&multicolumn2cinclude...&include...\ %Nice... Let's finish with the next row
multicolumn2cinclude...&multicolumn2cinclude
endtabular

Fourth image stops before 2nd column... Lets think... Do I need 5 columns?

3. 
   Third step... Let's see the vertical lines there? before reporting a bug... But I should!

Full code:

documentclassarticle
usepackagegraphicx
usepackagexcolor
begindocument
begintabularc
1&2&3&4\
 includegraphics[width=35mm]example-image-a & multicolumn2cincludegraphics[width=35mm]example-image-b & includegraphics[width=35mm]example-image-c \
multicolumn2includegraphics[width=35mm]example-image-a & multicolumn2cincludegraphics[width=35mm]example-image-b
endtabular
enddocument

They need the bug report... But...

Ok... I think I got it:

The second column has no the width of the third... Of course... No width needed there... Lets add a width:

... Ok I will add centered 17.5mm in both the middle columns:

documentclassarticle
usepackagegraphicx
usepackagearray
begindocument
begintabular
 includegraphics[width=35mm]example-image-a & multicolumn2cincludegraphics[width=35mm]example-image-b & includegraphics[width=35mm]example-image-c \
multicolumn2includegraphics[width=35mm]example-image-a & multicolumn2cincludegraphics[width=35mm]example-image-b
endtabular
enddocument

Why me??? Let's ask at stackexchange... I am out of here!

4. No more steps: Some explanation:

Let's add the 1&2&3&4 row back there:

documentclassarticle
usepackagegraphicx
usepackagearray
begindocument
begintabular
1&2&3&4\
 includegraphics[width=35mm]example-image-a & multicolumn2cincludegraphics[width=35mm]example-image-b & includegraphics[width=35mm]example-image-c \
multicolumn2includegraphics[width=35mm]example-image-a & multicolumn2cincludegraphics[width=35mm]example-image-b
endtabular
enddocument

Perfect... But what happened?

LaTeX used just 3 columns since the maximum of the cells needed there was 3 per row... So, the first column was a single column and not really a multicolumn... By adding these help numbers... we are actually lucky and found out what happened... So, adding a full row with empty cells (but all the cells) can solve the problem... Then remove the extra vertical space of the line with [-1em] ... Also, with some simple thoughts the second row cells should not be centered because of the extra width of the first and last column... May be r and l is a better choice ... and a tabcolsep fixed width would help... same distance too (as tabcolsep between rows) and:

documentclassarticle
usepackagegraphicx
usepackagearray
begindocument
setlengthtabcolsep1mm
begintabularc>centeringarraybackslashp17.5mm>centeringarraybackslashp17.5mmc
&&&\[-1em]
 includegraphics[width=35mm]example-image-a & multicolumn2cincludegraphics[width=35mm]example-image-b & includegraphics[width=35mm]example-image-c \[1mm]
multicolumn2rincludegraphics[width=35mm]example-image-a & multicolumn2lincludegraphics[width=35mm]example-image-b
endtabular
enddocument

But why didn't made it in a simpler tabular (of 3 columns)?

documentclassarticle
usepackagegraphicx
begindocument
setlengthtabcolsep1mm
begintabularccc
includegraphics[width=35mm]example-image-a& includegraphics[width=35mm]example-image-b&includegraphics[width=35mm]example-image-c\[1mm]
multicolumn3cincludegraphics[width=35mm]example-image-ahspace2mmincludegraphics[width=35mm]example-image-b
endtabular
enddocument

Nice... Clever!

But @egreg's answer is more clever!!! At least I found the problem!

PS: My "double post" needs moderator attention ... but I think it is an exception and have to offer to this site since the first question was about placing figures and I just found out I have something to say here that didn't found somewhere else and it is useful and about tabulars with multicolumn... If one of the two posts should be deleted I suppose that deleting the first one (my answer on the original question) would be a better choice because here seems more useful for me. So, please (reviewers) add a comment before flagging for deletion.