rectilinear (adj.)

• direct
• even
• flat
• horizontal
• level
• lineal
• linear
• rectilineal
• right
• smooth
• straight
• streamlined
• true
• unbending
• unbent
• unbowed
• unbroken
• uncurved
• undeviating
• undistorted
• uninterrupted
• unswerving
• unturned
• upright
• vertical