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In Call of Cthulhu, what's best practice for requiring skill checks when there's uncertainty about which skill would be required?

Looking at the [Call of Cthulhu 7E character sheet](https://www.chaosium.com/content/FreePDFs/CoC/Character%20Sheets/Character%20Sheet%20-%20base%20-%20Call%20of%20Cthulhu%207th%20Ed.pdf), I see two "investigator skills" that look very similar to me: "Elec Repair" and "Mech Repair". One is short for Electrical Repair, the other for Mechanical Repair.

The [skill descriptions](https://the-call-of-cthulhu.obsidianportal.com/wikis/skill-descriptions[]()) I found online say:

> Electrical Repair (10%) – Enables the investigator to repair or reconfigure electrical equipment such as auto ignitions, electric motors, fuse boxes, and burglar alarms. To fix an electrical device may require special parts or tools. Jobs in the 1920s may call for this skill and for Mechanical Repair in combination.

> Mechanical Repair (20%) – This allows the investigator to repair a broken machine, or to create a new one. Basic carpentry and plumbing projects can be performed. Special tools or parts may be required. This skill can open common household locks, but nothing more advanced. See the Locksmith skill. Mechanical Repair is a companion skill to Electrical Repair, and both may be necessary to fix complex devices such as an auto or an aircraft.

If I were to have my character attempt to, say, repair a wire for a radio antenna - something I have IRL experience with and know does not, largely, require electrical knowledge - would I use Electrical Repair or Mechanical Repair for this? A combined skill roll where I must roll under the lower percentage to succeed?

Generally speaking, **when there's a question as to which skill would be required, should the Keeper ask for a combined skill roll, choose one and require a skill roll, or require skill rolls for both skills**?