 In present-day English there are two variants of R liaison. In the case of linking an orthographically present R is pronounced between two words when the second word starts with a vowel. So in a variety such as received pronunciation which is low on the roticity scale, words such as mother, car or here are all pronounced without a final R. But if the next word starts with the vowel we can get cases such as mother-in-law, car and bike or here in London. American English by contrast is highly rotic and we have an R even in isolation as in mother, car or here. So the linking R is a standard feature of it anyway. The phenomenon of intrusion or the intrusive R is confined to the non-rotic varieties of English such as received pronunciation. Here the phoneme R can be inserted between two vowels even if it is not represented orthographically. The condition is the final vowel of the first item must be a low back or a central vowel such as R or or the central vowel schwa and the second word must start with a vowel. So in RP we can get india in the 19th century, law and order, bra and shirt and so on. Similar to most other connected speech effects such as assimilation and elision, liaison is mainly optional.