Why two miter slots?

MLCS T-Track Miter Slot Slider Bar are made from anodized aluminum and allow you to build a variety of jigs and fixtures for your shop

My new sled has some nice features like t-track for stop blocks, a hardwood top and aluminum runners for the miter slots! runner is in the miter slot

miter slot bar Made in the USA Custom fits standard 34 x 38 mitre slots; Slot Width Range:

miter slot track been thinking about building a cross-cut sled for my table saw and eventually and miter splining jig anyways, the runners miter slot (same