slightly before, or slightly after, the movement of the fingers). In such positions, the thumb is often thought of as an 'anchor' whose location defines what position the player is in. In very high positions, the thumb is unable to move with the fingers as the body of the instrument gets in the way. Instead, the thumb works around the neck of the instrument to sit at the point at which the neck meets the right bout of the body, and remains there while the fingers move between the high positions. A note played outside of the normal compass of a position, without any shift, is referred to as an extension. For instance, in third position on the A string, the hand naturally sits with the first finger on D and the fourth on either G or G. Stretching the first finger back down to a C, or the fourth finger up to an A, forms an extension. Extensions are commonly used where one or two notes are slightly out of an otherwise solid position, and give the benefit of being less intrusive than a shift or string crossing. The lowest position on the violin is referred to as "half position". In this position the first finger is on a "low first position" note, e.g. B on the A string, and the fourth finger is in a downward extension from its regular position, e.g. D on the A string, with the other two fingers placed in between as required. As the position of the thumb is typically the same in "half position" as in first position, it is better thought of as a backwards extension of the whole hand than as a genuine position. The upper limit of the violin's range is largely determined by the skill of the player, who may easily play more than two octaves on a single string, and four octaves on the instrument as a whole. Position names are mostly used for the lower positions and in method books and etudes; for this reason, it is uncommon to hear references to anything higher than seventh position. The highest position, practically speaking, is 13th position. Very high positions are a particular technical challenge, for two reasons. Firstly, the difference in location of different notes becomes much narrower in high positions, making the notes more challenging to locate and in some cases to distinguish by ear. Secondly,