For example i create a png file rappresenting the letter A of a particular font.
I use this command: convert -font arial.ttf label:a a.png
I obtain this png:

Then i have to elaborate this png and at the end i have to join more png (char) using the comand:
montage a.png b.png -mode Concatenate -tile x1 ab.png
if i view the joined png (ab.png) with ImageMagick Display i note that there are two rows and not one.

I think that the bottom row is the text field of png. The problem is that I would like to have a single row and not double. Where is the mistake? I suppose in the convert command