non-restrictive relative clause is typically set off by commas, whereas a restrictive relative clause is not, but this is not a rule that is universally observed. In speech, this is also reflected in the intonation. Writers commonly use which to introduce a non-restrictive clause, and that to introduce a restrictive clause. That is rarely used to introduce a non-restrictive relative clause in prose. Which and that are both commonly used to introduce a restrictive clause; a study in 1977 reported that about 75 per cent of occurrences of which were in restrictive clauses. H. W. Fowler, in A Dictionary of Modern English Usage of 1926, followed others in suggesting that it would be preferable to use which as the non-restrictive (what he calls "non-defining") pronoun and that as the restrictive (what he calls defining) pronoun, but he also stated that this rule was observed neither by most writers nor by the best writers. He implied that his suggested usage was more common in American English. Fowler notes that his recommended usage presents problems, in particular that that must be the first word of the clause, which means, for instance, that which cannot be replaced by that when it immediately follows a preposition (e.g. "the basic unit from which matter is constructed") – though this would not prevent a stranded preposition (e.g. "the basic unit that matter is constructed from). Style guides by American prescriptivists, such as Bryan Garner, typically insist, for stylistic reasons, that that be used for restrictive relative clauses and which be used for non-restrictive clauses, referring to the use of which in restrictive clauses as a "mistake". According to the 2015 edition of Fowler's Dictionary of Modern English Usage, "In AmE which is 'not generally used in restrictive clauses, and that fact is then interpreted as the absolute rule that only that may introduce a restrictive clause', whereas in BrE 'either that or which may be used in restrictive clauses', but many British people 'believe that that is obligatory'". Subjunctive The subjunctive mood is commoner in colloquial American English than in colloquial British English. Writing Spelling Before the early 18th century English spelling was not standardised. Different standards became noticeable after the publishing of influential dictionaries. For the most part current BrE spellings follow those of Samuel Johnson's Dictionary of the English Language (1755), while AmE spellings follow those of Noah Webster's An American Dictionary of the