mid·line 
(mĭdlīn′)
(mĭdlīn′)n.
A medial line, especially the medial line or plane of the body.
- double-header
- doubleheader
- double helix
- double-hung window
- double indemnity
- double jeopardy
- double-jointed
- double knit
- double-knit
- double negative
- doubleness
- double-park
- double-parker
- double play
- double pneumonia
- double possessive
- double-quick
- doubler
- double reed
- double-reed
- double refraction
- double replacement
- double reverse
- double rhyme
- double salt