noun, noun which may take the genitive case particle 'no'
Refers to a diagonal structural member or bracing, such as in carpentry or framework. Also used adjectivally with の to mean 'diagonal'.
壁に筋交いを入れて補強した。
We reinforced the wall by adding diagonal braces.
筋交いの線を引いてください。
Please draw a diagonal line.
筋違い (すじちがい) means 'wrong' or 'unreasonable', not 'diagonal'. The kanji 筋違い can also be read すじかい, but that reading is rare and easily confused.
Compound of 筋 (すじ, 'line; fiber') and 交い (かい, from 交う 'to cross; to intersect'). The exact historical derivation is uncertain, but the word has long been used in carpentry and construction for diagonal bracing.