It’s essential that crampons are fitted only to stiffened boots (often referred to as B1, B2 or B3 following a ratings system devised by the mountaineer Brian Hall). Fitting stiff crampons onto flexible boots could lead to the crampons falling off and disastrous consequences.
For ice climbing, choose a pair with vertical front points. An ideal set-up would be a pair of B3 rated mountain boots together with C3 technical crampons. The more aggressive front points give extra bite. These styles can also often be adapted to a mono-point set up for even better penetration and precision on hard ice. Attachment is usually by a metal front bar and rear heel lever which will only work on a B3 boot.
When mountaineering – perhaps doing the easier Scottish winter gullies or alpine routes – a C2 crampon will be technical enough for the climbing while still being relatively low-profile. The attachment system will be a rear heel lever and front toe cage, strapped together on a B2 or B3 boot.
For snowy walking, such as winter Munro bagging or on the easiest routes on Mont Blanc, a C1 crampon will be ideal. These styles strap on and flex with B1 rated boots, though they can also be fitted to stiffer B2 or B3 boots. They have low-profile points which give excellent grip on snow and neve but are also easy to walk in.