Delitos graves, menos graves y leves are the main, overarching categories of offences in Spain. In case of doubt, please refer to article 13 of Spain's Penal Code where they are specified.
https://www.boe.es/buscar/act.php?id=BOE-A-1995-25444&tn=1&p...<p><p>
There is of course a difference in how the offences are defined and calculated, but we are comparing the main categories of offences with Spain to the main categories of offences in the UK.
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So in response to Wendy's question, both legal systems classify their offences in 3 major categories, which are are similar in content (although the approach differs).
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The differences in legal systems do not take away from the fact that the three major crime classifications are similar in both systems, and are defined as the major crime categories by their respective legal bodies.
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I would absolutely recommend a footnote explaining they are equivalences and not direct translations (and why), but I do not feel that a good way of getting around this inconvenience is by using terms that already have a differing existing use in the target language - hence my proposal.