After hypothesizing that certain languages are related, linguists then try to build backwards and reconstruct the original language. They know that languages change systematically over time and by studying mostly written texts can come very close to describing a language that many not even be in use anymore. The sounds of words in a language may change over time. Sounds may be added over time as can be seen in the Spanish word for school, escuela which has its roots in the Latin word for school, scola . The addition of sounds like e in escuela is known as excrescence .
Assimilation is another example of how languages change phonetically.
Originally, the word heaven was pronounced more like hea v en . The opposite, dissimilation can also occur as can be seen as the French word marbre became marble after being incorporated into the English language. The pronunciation of the second r became less similar to the first. Metathesis , where sounds are interchanged also occurs over time. The English horse was once hros in Old English, and the Old English brrid became our modern day bird .