Around here, this site is a decent worldwide reference. Here's the Spain page with your stamp:https://www.stampworld.com/en/stamp...ps/1920-1929
Note that as with other stamp issues, and just to confuse us, the overall size of the stamp does vary, sometimes by a lot. So the margins will vary also. Note just the width difference between the 10c and the 40c below it.
You just have to see a bunch of stamps from a specific issue to get some idea of the typical range of margin sizes. Multiples like blocks or pairs will show you typical stamp to stamp spacing. Luckily, as production equipment got way better in very modern times, there is less variation in margin size for stamps with the same design. So:
The block shows us the typical spacing between stamps. There's a right sheet margin here, but it's been miscut so we get little bits of the pane at the right and the (full) margin between. How do we know? We've seen typical panes from the Liberty series or this specific 4c stamp to tell us something is "wrong". With that margin part showing perfs on both vertical (margin) sides, this is called a guttersnipe
, essentially straddle pane but more specifically for stamps with the full perforated margin.
After all that, straddle pane stamps aren't that big of a deal for collectors so don't get too wrapped around the concept. It's not often found but not particularly valuable. In fact they're disliked by many, being off-center and often straight-edged. It's a variety that is not catalogued. They will often be straight edged, but not all straight edge stamps, showing a big margin or not, are straddle pane stamps.
Now here's an old Great Britain stamp with a huge left margin. But from experience and seeing multiples, we know this is just how margin stamps were purposely made for GB issues in this time frame. Making your head spin yet?
Also note when you use Stampworld, pay attention to the sidebar at left. Not only are pages broken down by dates, but there are also separate pages for things like airmail issues, postage dues, other special use stamps. Bookmark this and maybe sign up (free), as the site will ask for login sometimes.
EDIT: Scott is a general catalog and as such doesn't go into such detail as sheets vs. panes. Their US specialized will show common sheet formats and give sheet and pane sizes. Not every country's specialized catalogs will give that info.
"Compound" mean a combination of the different perf gauges given, for example meaning (say) perf 11 horizontally, perf 13 vertically on a stamp as well as other combinations exist. But there may even be something like perf 11 on 3 sides and perf 13 on another. Most collectors don't care about the different combinations but specialists might.