In Indo-European languages, the words meaning "to be" are sometimes similar to each other. Due to the high frequency of their use, their inflection retains a considerable degree of similarity in some cases. Thus, for example, the English form "is" is a cognate of German "ist", Latin "est", Persian "ast" and Russian "jest'", even though the Germanic, Italic, Iranian and Slavic language groups split at least 3000 years ago. The origins of the copulas of most Indo-European languages can be traced back to four Proto-Indo-European stems: "*es-" ("*h1es-"), "*sta-" ("*steh2-"), "*wes-" and "*bhu-" ("*bʰuH-").