Tuffier's line is commonly used as a landmark for estimating lumbar vertebral levels during spinal anesthesia. However, its accuracy can vary depending on factors such as age, sex, BMI, and posture. This study evaluates the consistency of an alternative landmark, the "ASIS line" (a line connecting both Anterior Superior Iliac Spines), for identifying lumbar levels in the lateral decubitus position. Researchers will mark both the ASIS line and Tuffier's line on participants and use ultrasound to verify the exact vertebral level where each line crosses the spine. The goal is to determine if the ASIS line provides a more consistent and reliable estimation of lumbar levels compared to Tuffier's line.

Inclusion Criteria:

• Adults aged 18 to 85 years
• Patients scheduled for spinal anesthesia or lumbar ultrasound
• Participants who voluntarily agreed to participate and provided written informed consent

Exclusion Criteria:

• History of spinal surgery or spinal deformity
• Infection or wounds at the puncture site
• Inability to maintain the lateral decubitus position
• Congenital lumbosacral transitional vertebrae (e.g., sacralized L5 or lumbarized S1)
• Any other condition deemed unsuitable for the study by the investigator

Patients visiting the study centers (Kangbuk Samsung Hospital or St. Vincent's Hospital) who are scheduled for spinal anesthesia or lumbar ultrasound.