Generally, you need a special guitar adapter to connect your guitar to a Mac, or an external audio card. These devices turn the guitar sound into a data that can be read by your Mac.
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Tonebridge compatible devices:
• iRig, iRig 2, iRig HD, iRig Pro
• Apogee JAM, Line 6 Mobile In, Line 6 SonicPort, iRiffPort, Alesis IO Dock
• Ampkit Link
• Any external audio card
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To know more, see a detailed step-by-step tutorial we've created:
​https://www.ultimate-guitar.com/article/blog/how-to-connect-guitar-to-mac-device