This is an excerpt from Infant Motor Development by Jan P. Piek.
In order to sit unsupported, the infant must achieve appropriate trunk and head control. By five months of age, most infants can sit unsupported but cannot sit upright. The trunk is supported over, or aligned with, the pelvis and legs, which form a relatively stable base of support. However, an infant at this stage (figure 4.3; the infant in this figure is 22 weeks of age) shows an exaggerated forward lean necessary to support the head.
Around six to eight months of age, the upper half of the body has gained adequate control to enable the infant to sit upright unsupported. This is an important achievement as it allows the infant to free her arms and hands for exploration, as can be seen in figure 4.4.
This is an excerpt from Infant Motor Development.