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?

Examples:

bar         
beg        
occur      
permit    
patrol    

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’).

Travel

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 
parallel                       parallel 
paralleling                  parallelling 
paralleled                   parallelled 

Example:

The vetting service from Future Perfect is unparallelled.