Linking R and intrusive R are sandhi phenomena wherein a rhotic consonant is pronounced between two consecutive vowels with the purpose of avoiding a hiatus, that would otherwise occur in the expressions, such as tuner amp, although in isolation tuner is pronounced the same as tuna /ˈt(j)uːnə/ in non-rhotic varieties of English. These phenomena occur in many of these dialects, such as those in most of England and Wales, parts of the United States, and all of the Anglophone societies of the southern hemisphere, with the exception of South Africa. In these varieties, /r/ is pronounced only when it is immediately followed by a vowel.

Linking R and intrusive R may also occur between a root morpheme and certain suffixes, such as -ing or -al. For instance, in words such as draw(r)ing, withdraw(r)al, or Kafka(r)esque.

These phenomena first appeared in English sometime after the year 1700.

By definition, non-rhotic varieties of English pronounce /r/ only when it immediately precedes a vowel. This is called r-vocalisation, r-loss, r-deletion, r-dropping, r-lessness, or non-rhoticity.

For example, even though the word tuner is spelled with an ⟨r⟩ (which reflects that an /r/ was pronounced in the past), non-rhotic accents do not pronounce an /r/ when there is no vowel sound to follow it. In contrast, speakers of rhotic dialects, such as those of Scotland, Ireland, and most of North America (except in some of the Northeastern United States and Southern United States), always pronounce an /r/ in tuner and never in tuna so that the two always sound distinct, even when pronounced in isolation. Hints of non-rhoticity go back as early as the 15th century, and the feature was common (at least in London) by the early 18th century.

In many non-rhotic accents, words historically ending in /r/ (as evidenced by an ⟨r⟩ in the spelling) may be pronounced with /r/ when they are closely followed by another morpheme beginning with a vowel sound. So tuner amp may be pronounced [ˈtjuːnər æmp]. This is the case in such accents even though tuner would not otherwise be pronounced with an /r/. Here, "closely" means the following word must be in the same prosodic unit (that is, not separated by a pausa). This phenomenon is known as linking R. Not all non-rhotic accents feature linking R. South African English, African-American Vernacular English and non-rhotic varieties of Southern American English are notable for not using a linking R.

The phenomenon of intrusive R is a reinterpretation of linking R into an r-insertion rule that affects any word that ends in the non-high vowels /ə/, /ɪə/, /ɑː/, or /ɔː/; when such a word is closely followed by another word beginning in a vowel sound, an /r/ is inserted between them, even when no final /r/ was historically present. For example, the phrase bacteria in it would be pronounced /bækˈtɪəriərˌɪnɪt/. The epenthetic /r/ can be inserted to prevent hiatus (two consecutive vowel sounds).

In extreme cases, an intrusive R can follow a reduced schwa, such as for the example if you hafta[r], I’ll help and in the following examples taken from the native speech of English speakers from Eastern Massachusetts: I’m gonna[r]ask Adrian, t[ər]add to his troubles, a lotta[r]apples and the[r]apples. A related phenomenon involves the dropping of a consonant at the juncture of two words and the insertion of an r in its place. Sometimes this occurs in conjunction with the reduction of the final vowel in the first word to a schwa: examples of this are He shoulda[r]eaten.