In English, unlike other Germanic languages, the main verb cannot move to the beginning of a clause, with the exception of main verb be.21.6 In a GB account of inverted yes/no questions, the tense feature is said to be in C0 at the front of the sentence. Since main verbs cannot move, they cannot pick up the tense feature, and do-support is again required if there is no auxiliary verb to perform the role. Sentence ((405)) shows that do does not interact with other auxiliary verbs, even when in the inverted position. In XTAG, trees anchored by a main verb that lacks tense are required to have an auxiliary verb adjoin onto them, whether at the VP node to form a declarative sentence, or at the S node to form an inverted question. Do selects the inverted auxiliary trees given in Figure 21.4, just as other auxiliaries do, so it is available to adjoin onto a tree at the S node to form a yes/no question. The mechanism described in section 21.3.1 prohibits do from co-occurring with other auxiliary verbs, even in the inverted position.