existence of lexical or phonemic exchange errors provides evidence that speakers typically engage in forward planning their utterances. It seems that before the speaker starts speaking the whole utterance is available. Anticipation Target: Take my bike. Error: Bake my bike. Perseveration Target: He pulled a tantrum. Error: He pulled a pantrum. Performance errors supply evidence for the psychological existence of discrete linguistic units. Speech errors involve substitutions, shifts, additions and deletions of segments. "In order to move a sound, the speaker must think of it as a separate unit." Obviously, one cannot account for speech errors without speaking of these discrete segments. They constitute the planning units of language production. Among them are distinctive features, phonemes, morphemes, syllables, words and phrases. Victoria Fromkin points out that "many of the segments that change and move in speech errors are precisely those postulated by linguistic theories." Consequently, speech errors give evidence that these units are psychologically real. One can infer from speech errors that speakers adhere to a set of linguistic rules. "There is a complex set of rules which the language user follows when making use of these units." Among them are for example phonetic constraints, which prescribe the possible sequences of sounds. Moreover, the study of speech error confirmed the existence of rules that state how morphemes are to be pronounced or how they should be combined with other morphemes. The following examples show that speech errors also observe these rules: Target: He likes to have his team rested. [rest+id] Error: He likes to have his rest teamed. [ti:m+d] Target: Both kids are sick. [kid+z] Error: Both sicks are kids. [sik+s] Here the past tense morpheme resp. the plural morpheme is phonologically conditioned, although the lemmas are exchanged. This proves that first the lemmas are inserted and then phonological conditioning takes place. Target: Don’t yell so loud! / Don’t shout so loud! Error: Don’t shell so loud! "Shout" and "yell" are both appropriate words in this context. Due to the pressure to continue speaking, the speaker has to make a quick decision which word should be selected. This pressure leads to the speaker’s attempt to utter the two words simultaneously, which resulted in the creation of a blend. According to Charles F. Hockett there are six possible blends of "shout" and "yell". Why did the speaker choose "shell" and not one of the alternatives? The speaker obeyed unconscious linguistic rules