A guide wire configured for intravascular insertion, the guide wire comprising: a core wire comprising, in a direction from a distal end of the core wire to a proximal end of the core wire: a first rounded portion that is located at a distal-most end of the core wire, a second portion configured to allow the core wire to preferentially bend in at least one plane that passes through a longitudinal axis of the core wire; and a third rounded portion; wherein: wherein the second portion comprises a first flattened portion (208B), a second flattened portion (208B), a third flattened portion (208B), and a fourth flattened portion (208B), wherein: - the first flattened portion (208B) and the second flattened portion (208B) are round to flat transition portions disposed between the rounded first portion and the third flattened portion 208B, - the third flattened portion (208B) has a substantially constant rectangular or square cross-section, - the fourth flattened portion (208B) is a flat to round transition portion disposed between the third flattened portion (208B) and the rounded third portion, and wherein from the first portion to the third flattened portion (208B), the cross-sectional area of the core wire decreases and the core wire becomes progressively more flat, and from the third flattened portion (208B) to the third rounded portion, the cross-sectional area of the core wire increases and the core wire becomes progressively more round. a core wire comprising, in a direction from a distal end of the core wire to a proximal end of the core wire: a first rounded portion that is located at a distal-most end of the core wire, a second portion configured to allow the core wire to preferentially bend in at least one plane that passes through a longitudinal axis of the core wire; and a third rounded portion; wherein: a first rounded portion that is located at a distal-most end of the core wire, a second portion configured to allow the core wire to preferentially bend in at least one plane that passes through a longitudinal axis of the core wire; and a third rounded portion; wherein: wherein the second portion comprises a first flattened portion (208B), a second flattened portion (208B), a third flattened portion (208B), and a fourth flattened portion (208B), wherein: - the first flattened portion (208B) and the second flattened portion (208B) are round to flat transition portions disposed between the rounded first portion and the third flattened portion 208B, - the third flattened portion (208B) has a substantially constant rectangular or square cross-section, - the fourth flattened portion (208B) is a flat to round transition portion disposed between the third flattened portion (208B) and the rounded third portion, and wherein - the first flattened portion (208B) and the second flattened portion (208B) are round to flat transition portions disposed between the rounded first portion and the third flattened portion 208B, - the third flattened portion (208B) has a substantially constant rectangular or square cross-section, - the fourth flattened portion (208B) is a flat to round transition portion disposed between the third flattened portion (208B) and the rounded third portion, and wherein