this pattern is preserved in the modern past tense kept (similarly crept, wept, leapt, left). Verbs such as peep, which have similar form but arose after the Vowel Shift, take the regular -ed ending. The force of analogy tends to reduce the number of irregular verbs over time, as irregular verbs switch to regular conjugation patterns (for instance, the verb chide once had the irregular past tense chid, but this has given way to the regular formation chided). This is more likely to occur with less common verbs (where the irregular forms are less familiar); hence it is often the more common verbs (such as be, have, take) that tend to remain irregular. Many irregular verbs today have coexisting irregular and regular forms (as with spelt and spelled, dreamt and dreamed, etc.). In a few cases, however, analogy has operated in the other direction (a verb's irregular forms arose by analogy with existing irregular verbs). This is the case with the example of catch given above; others include wear and string, which were originally weak verbs, but came to be conjugated like the similar-sounding strong verbs bear and swing. In American English, the regular forms of verbs account for more than 90% of instances, whereas British English has a regular form frequency of 69% according to a 1991 study. The verb forms described in this article are chiefly those that are accepted in standard English; many regional dialects have different irregular forms, such as American English sneak–snuck and dive-dove as opposed to standard sneaked and dived respectively. In particular, it is fairly common in some types of non-standard speech to use (standard) past tenses as past participles, and vice versa; e.g. "have went" instead of "have gone" in Southern American English. Groups The irregular verbs of Modern English form several groups with similar conjugation pattern and historical origin. These can be broadly grouped into two classes – the Germanic weak and strong groups – although historically some verbs have moved between these groups. There are also a few anomalous cases: the verbs be and go, which demonstrate suppletion; the verb do; and the defective modal verbs. Strong verbs Many irregular verbs derive from Germanic strong verbs, which display the vowel shift called ablaut, and do not add an ending such as -ed or -t for the past forms. These sometimes retain past participles with the ending -[e]n, as in give–gave–given