Is it travelled or traveled?

When adding -ing and -ed to verbs, we sometimes double the consonant beforehand. People are often confused with ‘travelled/traveled’, ‘benefitted/benefited’, ‘focussed/focused’ and ‘targetted/targeted’. This tip answers some of those queries.

The official requirements are that we ‘double a single consonant letter at the end of any base where the preceding vowel is spelled with a single letter and stressed’.

What does this mean in practice?

Source: FuturePerfect