Meditation instructions from many traditions describe how to sit in considerable detail. The specificity is not ceremonial, and the reasons offered are consistent across sources that share no history.
Posture regulates alertness
Sitting upright and reclining produce different states of arousal, and the body reads its own position as information about what is about to happen.
A reclined position is associated with sleep by long habit, and practitioners who lie down commonly report drifting rather than settling, whatever their intention was.
Traditional instructions address this directly, prescribing a straight spine and an unsupported back precisely because the arrangement is difficult to fall asleep in.
Stability determines session length
A posture that requires continuous small adjustments consumes attention, and each adjustment is an interruption to whatever the practitioner was doing.
Cross-legged sitting on a raised cushion is favoured in several traditions because the arrangement forms a wide base and the weight settles without muscular effort.
Chairs work on the same principle when the feet are flat and the back is unsupported, and most contemporary teachers treat the two as equivalent rather than ranking them.
The chest and abdomen need to be free
Practices that attend to breathing require breathing that is unobstructed, and a slumped position compresses the abdomen and restricts the diaphragm.
Instructions about lifting the sternum or lengthening the spine are usually describing this effect, though the traditional vocabulary is rarely anatomical.
The change is noticeable within a few breaths, which is why teachers often ask a student to alternate between two positions rather than explaining the difference.
Hands and eyes are specified for consistency
Most traditions prescribe where the hands rest and whether the eyes are closed, lowered or open, and the prescriptions differ between them.
The common reasoning is that these details are otherwise decided repeatedly during a session, and each decision costs attention that the practice is meant to be gathering.
Fixing them removes a small recurring question, and the particular answer matters less than that it stops being reconsidered every few minutes.
Adaptation is standard rather than exceptional
Traditional postures assume flexibility that many people do not have, and injury or pain makes some of them unavailable entirely.
Teachers in most lineages have long modified the instructions for older or unwell practitioners, and lying down is explicitly permitted in several sources where sitting is not possible.
Anyone with a back, hip or knee condition should adapt with guidance from a clinician rather than persist through pain, since discomfort maintained deliberately teaches the body to associate practice with strain.