For a Tour du Mont Blanc start, Chamonix is the base and Les Houches is the trailhead. Most people who plan a Chamonix Tour du Mont Blanc sleep in Chamonix the night before and set off from Les Houches the next morning. Les Houches is the usual start of the standard route, which runs anticlockwise through France, Italy and Switzerland in 11 stages and closes back at Les Houches. Chamonix town is not a stage end: stage 10 stops at Refuge de la Flégère above the valley, and stage 11 carries on to Les Houches.
Because it is a loop, you can also start elsewhere. The usual alternatives are Les Contamines in France, Courmayeur in Italy and Champex in Switzerland. Starting at Les Houches keeps the classic order that most guidebooks and hut bookings assume: two French stages, the crossing into Italy on day three, Courmayeur on day four.