Is it travelled or traveled?
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?
It is true to say that there is usually no doubling when the preceding vowel is unstressed (‘enter’ becomes ‘entering/entered’; ‘visit’ becomes ‘visiting/visited’) or when the preceding vowel is written with two letters (‘tread’ becomes ‘treading/treaded’).
However, with some final consonants, even in cases when the preceding vowel is unstressed (so you would think that there would be no doubling), doubling does occur in standard received British English (but is not favoured in American English), so ‘travel’ becomes ‘travelling/travelled’. Others in this grammatical group (verbs ending in an unstressed vowel, followed by the letter ‘l’) are ‘cancel’, ‘counsel’, ‘dial’, ‘model’, ‘parallel’ and ‘signal’.
Some words change their spelling to cope (they add a letter ‘k’).
panic panicking panicked
traffic trafficking trafficked
frolic frolicking frolicked
bivouac bivouacking bivouacked
What about ‘focus’?
This word can take either double or single s, with the single option being highly preferred.
focus focusing/focussing focused/focussed
Here’s an odd one to end:
American British English
The vetting service from Future Perfect is unparallelled.